If you have any love for Diablo 2 stay away from Diablo 3. As an avid fan of the past Diablo games, in particular Diablo 2, Diablo 3 has been the biggest let down in gaming I have ever experienced. The only similar aspects that Diablo 2 and 3 share are that gold is still worthless, you can play the barbarian class, you kill monsters, and that's where the similarities end.
In terms of comparison between Torchlight 2 and Diablo 3, it is a very similar to the argument between Diablo 2 and 3. Honestly, I haven't tried to compare the games directly. I've only compared them in terms relative to Diablo 2 as a base line of what is an improvement and what isn't (if that makes any sense to you).
Take this next statement to heart: of the many reasons why Diablo 3 sucks, has failed in the eyes of its fans, or has been considered a horrible game, the issue revolves around the fact that Diablo 3 is NOT an "RPG." If you expect RPG elements in this game I can assure you that you WILL be disappointed. In my opinion, for what ever that is worth, Diablo 3 fails at being a "Diablo," game. There are tons of reasons as to why Diablo 3 is unworthy, and I can expand on them if you like.
However, I can sense that you'll want to experience this game regardless of what others have said about it. Hell, I would have been the same way. If someone on day one had told me exactly what this game would be like I would not have believed that person and purchased the game anyway.
I honestly would not pay more than $10.00 for Diablo 3, and that's being generous. If I were you I would seek out a friend that has quit playing, and just play on his account. That way it is free, and you won't ever get the feeling that you wasted your money.