For me, Diablo 3 is better by a long shot. While most believe that it doesn't live up to its predecessor (I'm still iffy on that) it still stays leagues ahead of the Torchlight series as a whole. One thing which surprised me about Torchlight 2 was its demo. They seemingly specifically designed the demo to convince people not to buy the game. I mean the whole thing is a mixture of 1 hit kills and vast expanses of nothing. Needless to say it got stale very quickly.
While Diablo 3 has some controversial features (Always online, Auction house) and controversial changes (Art style, Skill system), Blizzard manage to pull the off in a way that very few developers could do. The always online becomes irrelevant (except in a few special circumstances) when their high quality servers are factored in (negligible lag on shit tier Aussie net) and provides worth while enhancements (Seamless single and multi player). The auction house provides an easy to use way of trading, something which is needed in games like these.
The new art style and enhancements to the engine (or rebuild, idk) provide some awesome experiences. For a good idea of what I'm referring to, compare the start of Act 5 in Diablo 2 to Act 3 in Diablo 3. That shit is awesome. The new skill system allows greater flexibility and customisation while also encouraging experimentation. Blizzard really did a good job with this, most of the skills and their runes are well balanced making most of them viable options which then encourages build diversity (unlike in Diablo 2 where you are all but forced into "optimal" builds). The gameplay in general looks, feels and plays better than ever. Making every encounter highly enjoyable.
But that's enough ranting about Diablo 3. Just make sure you research them both well and also look at the alternatives like Path of Exile (which hits open beta soon), the upcoming Grim Dawn or some older (but still great) games like Titan Quest or Diablo 2.