Well, if you really wanted one, wouldn't you be well aware of it's capabilities? I say get the PSP, since you've just said you can do more with it, you must obviously know more about it, or, otherwise, you wouldn't be asking, you know which you'd want already. If you don't already have a PSP, get the DS, otherwise, you may want to re-think. 10 dollars isn't much.
You can always make efforts to find games with good reviews, and what-not. The only thing I've seen on DS, is 2D games, and mario games. Not that there's anything wrong with 2D, it seems to GBAish for me to enjoy for too long. I played Megaman ZX, and Bomberman (whichever one it was) both were ok, but, it felt like i was playing old school when i was expecting something else. Oh yeah, I played those games two years apart from each other....
Speaking from experience, Daxter, Megaman Powered Up, Maverick Hunter X, and Tekken: DARK RESURRECTION are all good games. You may also want to look into GTA:CTW, which is on both systems, but, superior on PSP. My PSP is used as a MP3 player 1st, and a gaming device second, and video/internet...








