Well, one is free for a start which is always a good thing. I recently played Path of Exile and it certainly kept me entertained. There are similarities with Diablo 3 in that you have to find better weapons/armour. (For some reason the original Diablo was the better than Diablo 3 as the weapons had names suiting their status, whereas Diablo 3 has cards and somehow that doesn't work. POE is more alike the original Diablo.)
I did play both games solo so perhaps in a group it might be more fun.
In POE you have to find gems to add to weapons/armour to increase their ability which may already be quite high. I liked the different colour weapons later on in the game (blue, yellow then white) denoting rarity. I loved that after a battle, arrows were embedded in everything. Overall I think I preferred POE. The Diablo series never improved over the original game. I personally do not think that added graphics is key to good games and POE shows this. Don't get me wrong, the graphics in POE was good but perhaps wasn't as good as Diablo 3 -- but who cares.
Diablo 3 became tedious in my mind with tasks that were identical between different characters. The weapon drops were getting better so you had a good chance against the opposition. In Diablo 3 I didn't like that characters had little overlap, an archer had very little ability as a magician, as an example. Some of the arrows became grenades and I really hated that. Exploding arrows OK but grenades??
POE had its downside and some were serious niggles. The programme bombed me out after defeating two big villains/bosses and the last one was the end of the game. On going back into the game, it hadn't logged that I had defeated him and I would have to start that episode again. The villains can be seriously hard and the final one almost impossible. In Diablo 3 the villains (bosses) died after a couple of arrows which was far too easy and on occasions I hadn't realised that I'd actually defeated a boss.
In POE some drops just didn't exist, for example I only ever found one jewel for the skills tree. The same was true for various cards. I also did my best to improve my skills against against fire, chaos etc, but on the skill tree they seemed spread out and difficult to find. The skill tree is enormous and I think there was even a skill on dealing with ingrowing toenails. I never did find a minion unless a golem is one.
I think I spent more time on POE than Diablo 3 (including the expansion pack (Reaper of Souls). For a free game it is epic and entertaining. For me I preferred POE and for a freebie that can't be bad.