Get whichever one you don't have to pay to play once you've bought it.

Or, better yet, don't get either, and wait for Diablo III.

**** yea, get Diablo III. And to not get bored waiting for Diablo III, get Diablo II.

I have Diablo 2 and Starcraft, i'm waiting for both of the new one's already.
I've played maybe 8 trials of WoW and meh, I had some fun. Havn't played Warhammer yet...Eh...

I think I might just get Counter Strike, Gmod and L4D instead >.>...