Pfft. DS PHAT ALL THE WAY!!!!
Hehe, I got my DS release week, and still haven't upgraded. Probably will get a DSi at some point when there's good DSi titles, and I largely finish playing GBA games. As it is now, I play GBA games on my DS about as much as I do DS games.
As for DSL vs DSi, it pretty much boils down to how much you play GBA games.