Grim Dawn is a way better game on almost all fronts as far as I am concerned.
But which one you would like more depends quite a bit on personal preference.

D3 is easy to get into and gear drops set your playstyle as skills are free to move around as much as you wish.
GD on the other hand works the other way around, a drop is great or useless junk depending on how you have built your character. While you can swap around spent skill points and shrines it is not free to do so.

I have not tried GD in multiplayer so far but I am pretty sure that D3 is on a whole different scale when it comes to playing with new random people. If you plan to play with select friends or solo then it only matters that D3 force you to always be online.

D3 wins when it comes to controls and fast paced combat, there is a fluidity to blizzard games that no other manage to beat and rarely match.
GD win big when it comes to decision making in combat, you use your skills and position to take out targets that either are dangerous or just the weak ones you can kill fast.
D3 combat is pretty much holding the button down and hitting everything on screen at the same time with no real aim needed.

D3 get repetetive and boring somewhat fast but they offset this with seasons where they balance class skills and gear a bit, usually to make something worthless that no one used in previous season the one thing everyone use in the new. Sadly D3 is all about the end game these days and it is a boring grind to reach it, even if you easily do hit max level in a day.
GD have a longer life due to it's massive class system where I spent some 200 hours just to try out the basic ones, never mind that you can combine any 2 classes.
I have yet to reach end game for GD so I can't really say how it compares but I would be ok to reach max level and be done with that character due to the journey to it.

Oh and if you love Diablo 1-2 then go GD because D3 would be better off if it was not compared with the old games and instead seen as a standalone.

This won me over, thanks friend :)

You can go with the Trial version of D3, it's free of course. IMO was boring didn't enjoy it, and you have to be always Online

The better points of D3 compare to GD are that D3 is more stable, with less small stuterings, and it is easier to play with friends.
However GD is being patched to correct its multiplayer game. The crashes of GD are not big bug for it's mainly happening only while closing the game, and the stutterings are not often (but can be ennoying for an ironman/harcore game).

For all the rest, GD is superior despite its old engine:
. D3 can be beautiful, but GD is even more (TitanQuest was already proving the beauty capability of its engine)
. The graphic effects of skills are stunning (yes even more than D3), and it's the first time I'm actually impressed by those effects in a game. However you have to reach level 60 to really understand this quality.
. The classe trees give true choices (not some make up like D3 for the game to be easier balanced).
. The combat have a better feeling (so no whines for GD like those were about TQ)
. Internet is not needed for the solo play (no server will forbid you for not being up).
. ...

This. Diablo 3 lost the "spirit" of the series, both in artwork/graphics/environment presentation and in gameplay state. It is an ALWAYS ONLINE DRM, no offline single player, disaster upon launch, with a blatant and shameless real money Auction House system used by Blizzard itself, which was dropped after they pimped up enough big money for thin air and received harsh criticism. Diablo 3 is a craptastic clusterfuck that puts the entire series in shame; it doesn't even hold a candle to 2, which is still, to this day, having a large playerbase. Diablo 3 is also oversimplified (no stats allocation, no character unique stats customization, not all players can wear all armors if they "shape up" their player towards that end), consolified (this needs pages of analysis to cover fully and get the message in detail along) and a sorry sight for old gamers and veterans alike...

Get Titan Quest and Grim Dawn. They are the best Diablo 2 clones out there, they still retain complexity in character creation and customization (taking it huge steps further ahead, with dual classes and complex trees with surgical precision planning of talents) and take it to the next level. Grim Dawn in short, is the *real* diablo 3. Or something like that. Or something along those lines.

Grim dawn doesnt have any cool skills.. Diablo 3 has a lot!!

I'm okay that the animations of the basic skills are slightly better in D3. But once you've reached lvl 60+ in GD, then its skills are great, mainly from the items, and they are the coolest I've seen in an H&S.

Good for you but I havent found a single build that is more fun than I expected.

Titan quest had at least 4-5builds that were enjoyable. This game? not even a single one :|

Path of Exile. But this is good too.

Not if you want some single player action.

My choice is GD by a country mile.

My choice would be GD , though i don;t like the tomboy look of the female char. Don't dig short hair for females. Even in Titan Quest the female char looks better.
But the atmosphere is great, almost just as dark as the old diablo games. Diablo 3 is just World of Warcraft with diablo chars and lore added.

Path of Exile is mostly singleplayer.
Yes, PoE is an online game and requires internet to play, because there is also a PvP mode in it.
However, PvP is optional and you can just play the game in PvE mode.
I play PoE myself solo and don't bother much for the PvP stuff,also because PvP is rather on a small scale.
You have no guilds, no instances and the like.

Grim Dawn because it can be played offline :)

Torchlight 2 is also a more faithful D2 clone, including some original D2 core staff. Blizzard is just a naming rights owner.

Something about the controls in Titan Quest and Grim Dawn feels a little stiff to me. Grim Dawn is better, but still not as fluid as Torchlight 2 or League of Legends. It's hard to put my finger on it, but it just feels a little sluggish. Maybe it's the visuals, or maybe it's some subtle quirks of the animations and skill/attack cancelling. But it just feels slightly stiff to me every time. (D3 is very fluid, at least when it's not lagging to hell and back. And D3 has level scaling monster, which means that as you outgrow your gear, the monsters outgrow your gear along with you. You only begin to accumulate an actual advantage by stacking bonus points from "prestige levels", which is extremely silly and causes life to begin at 80.)

Overall, Titan Quest and its spiritual sequel Grim Dawn (headed by a lead designer/programmer for TQ) are extremely worthy and bring a lot to the table in the form of their "choose two masteries" approach to constructing classes. The character building mechanics are very punishing of waste, but they're much more familiar to Diablo 2 players than D3's "you are what you wear" and "life begins at 80" philosophies.

Torchlight 2 has a lot of modding capability, though it's not the smoothest or best documented system.

Anyway, D3 squandered a lot of good will and enthusiasm and should be avoided IMO.

I am actually a complete newbie to the Action RPG genre and bought both Grim Dawn and Diablo 3.

Diablo 3 was super satisfying and I enjoyed it immensely... Until I became super overpowered and walked through Act 2 without attacking anybody and still killed everything but boss monsters that came my way on hard difficulty. I am not exaggerating, I just got some sort of pants that do so much poison damage that everything around you dies without you touching it. This isn't skill, this is just braindead coasting through a somewhat dry story.

Grim Dawn was a super hard switch after that, considering that I couldn't kill 30 enemies in one hit. It's a bit frustrating at first, but I enjoy the difficulty! I am a pretty stupid player and probably make the worst decisions in terms of gear, but it's great that I am being punished for that to some extent (though it's been getting a lot easier).

I know you got your answer already from the first post, but still, though it'd be fun to throw my 2 cents in :)

Oh, and I kinda wish I had picked up Titan Quest instead; I just really love the setting for that game more than.. Well.. The grim dark. At least so far!