Having played both games extensively I will try listing five of the reasons why each one might appeal to you and also the two main weaknesses they have. I will not say which game is better, this is a strictly subjective matter, but in the end you'll have enough knowledge about both to choose for yourself.
Now, first and foremost lets outline the genre. It is Hack and Slash, in which you kill thousands of monsters repeatedly in hopes of getting better loot and increasing your power. The games in this genre have very light RPG elements and tons of replayability as long as you enjoy the slow grind, and accumulating of riches. They are not for everyone, but it is very possible for a person to play them for years without getting bored.
Let's start with Diablo 3.
Brand and Lore Recognition
Are you a fan of Blizzard and their games? Have you played the previous Diablo games? Do you like the lore and the world with its Heaven, and Hell representations? Even though there were some criticisms about the new story and graphic choices, Diablo 3 still has our lovable big red devil, some breathtaking locations and in the final act(before the expansion that is) you even get to fight through one very detailed Heaven in ruins. It is grand and it feels good compared to the completely new world Path of Exile presents us with. As far as the brand name goes, things did not start well enough but Blizzard, being the giant they are, did manage to fix most of their earlier mistakes and are patching the game regularly enough to instill a sense of stability into their customers.
Graphics and Fluidity
Some people complain the graphics aren't dark enough and the places are not very gothic looking. They are mostly just shallow complaints though. Darkness is not needed, we are not playing a horror game. The good lighting, however, does reveal the detail and polish Diablo 3 has, while keeping its sense of dread, especially in the last areas before finishing Act 1. The monsters are reasonably scary, some of them very big and imposing. But the biggest highlight is just how seamlessly everything works. Diablo 3 is online only, yes, but as long as you are having a nice stable connection you will have a pleasant time executing your skill combos. Path of exile in comparison, even after finally solving the desynch problem, still feels clunky, especially during location transitions and certain very colorful boss fights.
Feel of Power
One thing Diablo 3 consistently brings to its players is the feel of power once you get your gear and nail your build timings. We are not talking about puny thousands of damage here but billions! It comes with its own little inconveniences but as a whole you will feel like the god you are meant to be, regularly killing things, which were troublesome to you before, in one hit. Looking at the game as a whole, it has to be this way because of the next reason, and it is
Endless Endgame Content
The so called "endgame" has been a problem in this kinds of games. Take a normal single player RPG for example, you'll eventually hit the level cap and/or kill the final boss, and you'll be done. Not in Hack and slash, where keeping the player interested for hundreds of hours is the goal. Diablo 3 seems to have found the right way by introducing an endless dungeon of sorts, called "Greater Rift" . In it the monster levels, attack and defense scale to infinity, so it will always pose a challenge no matter how well equipped and played your chosen champion is. A common complaint, that greater rifts are boring because they are the only endgame content, has been addressed by an overall non-mandatory increase in difficulty when it comes to the other content, which will make people spend more time doing other things. As it stands, it does have the right bait to hook the players in and make them stay, and play.
A Generous Loot System
This was something introduced in the expansion, the new Loot 2.0. Trading was pretty much erased from the game, only existing in a very limited form, so it was necessary for the developers to shower people with great items to keep them interested and working towards improving their characters. The most ingenious addition, however, was the column of light, which signifies that another much desired item has dropped. Just seeing it feels like an accomplishment already, finding that it is an upgrade over your old gear is the cherry on top. It feels good to be rewarded for the time you spend slaying monsters, after all this is the reason you've been doing it all this time, and Blizzard managed to make it awesome.
Pretty good, solid reasons so far. Now let's jump into Path of Exile and see how a completely F2P game manages to sustain itself with cosmetic/quality of life micro transactions, while keeping a big and healthy community going.
A Layered Character Customization
Remember Diablo 3 with its skill point and stat distribution? Path of Exile took it one step further by introducing the mother of all skill trees. Yes, it is the same for every class, but considering how every class starts in a different part and has access to different nodes you can bet each one will have a different optimal play style. Not to mention that you are free to do crazy things like a witch wielding a two handed hammer and a barbaric marauder casting spells. I said layered though, the skill tree is only consisting of passives. The active skills themselves can be customized with various support skills, up to five of them, creating a very diverse and deep character progression and planning, something sorely missing from Diablo 3. It does also up the difficulty and learning curve though, so to some people it can be overwhelming and undesirable.
Meaningful Unique Items
The items in Path of Exile feel unique, exactly how they are supposed to be. Some of them do have only a bit of a beefed up generic stats, but the majority offer some unique and out of the box possibilities. Maybe the greatest part is how those powerful items usually come with a negative effect you must always take into consideration. That armor which gives you a great increase in your fire damage, for example, makes you very vulnerable to spells when you are at low life. The depth and thinking involved only gets that much intense when you need to juggle passive skill points, active skill effects(which most likely will put a strain on your mana) and the unique items, which can improve your build but also present you with new challenges because of their negative effects.
A Revolution in the Currency System
This one is what I consider the most amazing aspect of Path of Exile - gold doesn't exist. This universal currency is instead broken into various orbs, which you can either trade for gear or use to craft your very own items. It is ingenious and something, that will make your daily gear runs more thrilling for sure. You are not only looking for good items but also for good currency, which is not a chore to pick up the way gold is. It is worth to note that having the possibility to improve your items through orbs can be very devious. It is luck based and you can find yourself blowing all your currency for nothing, just the small things you need to learn about the game. Hack and Slash are mostly a gamble, so be prepared to think like a gambler.
Useful Things Drop Frequently
In Diablo 3 you'll hardly find yourself amazed by the scores of blue and yellow items that drop constantly. You only care about legendaries and sets. In Path of Exile every item can have a value, even low level whites. Because of the socket system and the way trading items for orbs at the vendors works, you can pick up a "useless" white item, which so happens to have 6 sockets on it, and trade it for some currency. In a similar fashion you can collect low level yellow items and sell them in a certain way to get an even higher currency for yourself. Once you reach the endgame interesting things will drop most of the time and they will be useful for something. This will make your play sessions less boring and will glue your eyes to the screen in search of some more items to trade for currency, which consequently will allow you the chance to improve your gear or to trade for something better. Lots of possibilities and they are all good ones.
Changing the Rules and Adding New Ones
Both Diablo 3 and Path of Exile have their seasonal leagues, which persist for about 3 months and offer the players an opportunity to start fresh. This, however, is only the tip of the iceberg in Path of Exile. There are one month long leagues, weekly leagues, race leagues, leagues with some extreme rules to provide additional fun, and rewards, of course. Hell, even the seasonal leagues have their own unique rules added to the pile, which are subject to testing and eventually added to the main game. Some of them at least. Either way, they do make the experience different, keep players constantly on the edge because of rewards and can even introduce their own unique builds because of the new way the game has been played. In this respect Diablo 3 is unfortunately very stale, offering only some new items for the seasonal fresh start players.
Ok, now that all the good things about the two games have been said, let us concentrate on the negatives, perhaps the most important part in any decision in regards to spending your free time.
Diablo 3 and Third Party Software
At this point the bots and other third party programs in Diablo 3 have become notorious. Because of the grindy nature of the game, bots thrive and make their owners much better at completing higher level challenges inside the greater rifts. Even though Blizzard cut off the trading exactly for this reason, botting never disappeared. Furthermore, a program similar to map hack has been gaining immense popularity in the very competitive leader board community. Blizzard's response to all this is silence and a few bans for the loudest offenders. As it stands, if you want to be the very best you can't do it without a dedicated guild and some third party programs to help you out unfortunately. At least the lack of PvP content kind of mitigates the need to be worried, but nonetheless unfair competition prospers.
Diablo 3 Has a Staggering Lack of Diversity
It all comes down to how the game is balanced. Or the lack of balance. You see, Blizzard has a very interesting philosophy when it comes to balancing classes and skills - item sets solve everything. Every now and then with a new patch they introduce a new bunch of sets, which improve a certain number of skills for each of the classes, changing them from almost useless to very very powerful. This is good, right? Except for the fact that the increase in power is so extreme you need to wear the item set and play with the skills it boosts. Oh, and the same item set usually has six or so pieces, which take up your gear slots AND dictate what skills you are going to use. This not only kills a huge chunk of diversity, it also makes it mandatory for players to hunt for these sets, otherwise they'll be very underpowered. This system has been called a band-aid and lazy numerous times, but Blizzard keeps at it and it is almost for certain that it won't disappear.
Path of Exile Has a Plethora of Cheap Mechanics Which Will Kill You
Oh yes, the infamous difficulty in Path of Exile. You really need to sit down and read about everything if you want to play it, simply leveling won't prepare you for some of the deadly encounters and/or amazingly damaging combinations you'll meet with during your exile. It is a byproduct of the numerous things the designers added to the game and how they can interact with each other for some lethal results. I'll give you an example. There are ghosts, creatures which prefer to run from you instead of attacking you directly and instead possess a high level monster, which gains some nasty side effects to its abilities. There are also rogue exiles, very fast and deadly versions of some of the more popular builds you can do in the game. And some of these exiles are tailored to counter a certain play style. Now imagine having a ghost inside the exile, who was designed to completely annihilate a character, who depends on evasion for their defense. The encounter can be ugly, usually resulting in a less than a second death out of nowhere. Difficulty spikes like this one are not at all uncommon and while they do keep you on your toes some of them are blatantly unfair. The thing is, the developers take pride in how hard their game is, just look at the last added act and its boss. What can I say, the game is brutal and not exactly as casual friendly as Diablo 3.
The Endgame Randomness and Clunky Trading in Path of Exile
This has been the gripe of many people throughout the years. The endgame content is gated through a rather large and nasty random number generator wall. It is maps of different levels you can change to make harder. The harder maps yield better rewards. Good so far, but you have to get these maps as a random drop, so even though you are perfectly capable of conquering the upper map levels, sometimes you'll be unlucky and find yourself without the appropriate high level maps in your possession. It is a drag and feels extremely artificial to lock up the content in such a way. Considering for how long the map system has persisted, maybe it is time to change it to something else. It is archaic. And talking about archaic, open up the trading channel and bask into the glory of constant spamming. There is also the good old haggling going on, when you try to wrestle a good price for something the other person doesn't even bother to price correctly. There have been some improvements with a special website, where you can look up items and their prices easily, but the trading still involves contacting the seller. Some smart trolls even learned how to abuse the system by intentionally posting great deals and not responding to messages for exchange. It sometimes feels like a waste of time to try and trade but on the other hand the game is built around trading, so you have to pick your poison and stick with it.
And there you have it, a comprehensive look and comparison between both games. Diablo 3 wins when it comes to better graphics, stronger developer, big numbers and accessible endgame. Path of Exile wins when it comes to depth and customization, random drops and flexible currency. Maybe the best part about it all is how both games manage to cover different aspects of gaming and spectrum of players, avoiding direct confrontations, and relying on their own strengths instead.