I recommend DS, though I still have an urge to get a PSP. Price isn't really as big of a deal anymore, since the games cost about the same, and the systems aren't far apart in price. But I find the DS games fun, and good for a time-waster. I'm afraid that I wouldn't be able to pick up and play a PSP as easily.
LEFT4DEAD411.COM
Bet with disolitude: Left4Dead will have a higher Metacritic rating than Project Origin, 3 months after the second game's release. (hasn't been 3 months but it looks like I won :-p )