Well, the main points I would like to bring up are
- Torchlight 2 is cheaper. It's 1/3rd of the cost of Diablo 3.
- Torchlight 2 can be played in single player offline as well as locally via LAN. Diablo 3 cannot.
- Torchlight 2 has full mod support. Diablo 3 does not.
Now, aside from those major factors, it's really a question of which game you prefer more. Both games are different in several key areas, so I would suggest trying both and deciding which one you like best. You can try Diablo 3 via a guest pass. The beta for Torchlight 2 is pretty much coming to an end so I doubt there's much chance of getting into that, unfortunately.
Personally I would like to say that I highly discourage you from buying Diablo 3 because I do not approve of the anti-consumer practice of always-on DRM, nor do I like how they designed core features of the game around the Real Money Auction House (RMAH for short), which is essentially where in-game items obtained by players are then sold for real-life cash to other players while Blizzard takes a percentage of the money.