Matter of preference bro.
PSP will sport some nice visuals, wonderful sound, plus works as a media center (depending on how big of a card you're willing to invest in - the further it will go), but has a MUCH lower battery life. Titles range as vast as the PS2 and have a ton of third party support - not to mention 20+ launch titles.
The DS on the otherhand hs less traditional game play from the two screens and one being a touch screen. Obviously less visual standout but the battery life is killer. The touch screen and 2 screens incorperate a ton of new gameplay features. The negative - lack of software.... now. Tons of software is coming out but as it stands theres only a handful of "worth owning" titles. But just wait til advance wars DS, FF3 remake, etc etc etc
Weigh out the pros, cons, and obviously price and battery life.
Hope you're happy with the choice you make.
-AlAdsit
xoxo