I got a PSP (free) as soon as they were released and it was pants. I played 2 good games in 4 months, Lumines and Everydodys Golf, things may have changed but personally I'd go for a DS, unless you already have one that is.
Btw, I traded my PSP in while it was still of value.
if you get a PSP get a version 1 one, as you can get free games from the not on em. if you get a DS then go to http://www.householdhacker.com as the teach you how basically turn it into an iphone ( ie turn it into a phone and media player).