DS, for one reason only: Nintendo knows the difference between original games for a platform, and porting games to a platform.
Sony wants people to be able to play console games on a handheld, which just isn't going to happen very well. Sure, they'll be decent, but after the console version it's just not as fun.
Nintendo on the other hand, doesn't give two craps about porting games, they're writing all original content for the DS, so everything is customized and optimized, and plays way better than anything on the PSP.