Do you want to play tons of great games, connect wifi, store pics, listen to music and play videos and movies, get homebrew and play tons of great PS1 games, view pictures directly from a memory card out of a digital camera, and share games with a friend even though only one of you has a copy?
Or do you want to play something with Mario in it all the time?
It really depends on what kind of games you like and what kind of gadgets you already have. I have a video iPod so I really would have no use for a lot of the features the PSP has plus I am a big mario fan so for me the DS was the best choice. And the DS has Brain Age so you can actually get some pratical use out of it. It all depends on you... oh and the DS can take Game Boy Advance games and you can play with other people, your friends or people from across the world. You can get web browser for the DS also. I love my DS and I wouldn't trade it for anything (and I only have 1 game so far, Brain Age)