I'd have to disagree with 198x, the DS has a far greater number of games covering the whole spectrum of genres. It is quite incorrect to say that the DS is limited for original gameplay, just look at the Warioware games or Brain Training or Nintendogs or Mario 64 DS, the list goes on and on.
I'm certainly no fan boy or hardcore gamer. I own both formats and I can honestly say that there have been more games that tickled my fancy on the DS than on the PSP.