I'd say go for the DS. Sure, the PSP has better graphics and can do more, but with the DS you get games like Trauma Center, Final Fantasy IV, Pokémon series, the Ace Attorney series, and plenty of nifty import games. The PSP has Final Fantasy Tactics and Disgaea, but the DS has Final Fantasy Tactics A2, and will be getting Disgaea in a couple of months.

It's all a matter of preference.