What branton said..

DS platform also has LOADS more games releases than PSP.
Though I think PSP games in general has higher production quality and is more like (full) console games rather than the games on DS platform.

But it all boils down to what genre of games you like the most. If you like more variety, then the DS would give you more bang for the buck.. it has plenty of casual games and the gameplay mechanism are much more varied than PSP due to its multiple inputs (touchscreen, camera, microphone, keypads).
If you want a PS2 in your pocket, then go with PSP.