To be honest, I think I have played both games for about 1,5 months, before shifting to other games. HOWEVER, in that time I have finished Diablo 2 about a zillion times (and damn, did I HATE that swamp!) which got quite repetetive, and BG 2 only once (really, stopped on my second run in the Underdark, when my brother redirected me to EQ
It's a bit hard to say, really ... Diablo 2 is all about action, little about RPG. BG is a lot about RPG, and not so much action, in respect to Diablo 2.
However, comparing Diablo 2 with Diablo 1, I think they did a bad job. I really liked the first game, and played it through maybe 20+ times, and still it was funny. The game's graphics looked rock-solid, the colors maybe a bit blant (sp?), but that made it look even better. Good ambiance, and highly detailed graphics. In Diablo 2, although they really did a good job on the interface, the graphics were a step back. Bright colors, sharp contrasts, and a somewhat 'blocked' appearance made it look worse (although on the higher resolutions this was not so much of a problem). Also, they introduced some monsters that were almost IMPOSSIBLE to kill. Like Duriel for a Sorceress, or that Flame Spider Boss in the Swamp for the Paladin. Not to mention Diablo himself for the Amazon.
I'm dubiously regarding the expansion of Diablo 2.
No, at the moment, the only 2 games that I look forward to are Warcraft III, and Commando's 2. Beyond that, I think I'll be on Everquest. Have been playing that game for a month, spending virtually all of my free time on it, and I yet have to get the feeling 'oh, BORING!'. I'm still enthusiastic about gaining yet another level, and looking forward to yet more spells (and they are more than with BG2