I'd say say go for the DS. I feel the PSP's library is quite unimpressive, while the DS has a nice large variety of games.
As an owner of both systems, I fully endorse this statement. The PSP is by far a more powerful system (in terms of graphics, media capabilities, and controller design), but I use my DS much more. I have a nice size stack or DS games that I love, but only a handful of PSP games that I enjoy. Also, the DS games are generally cheaper on the high end, and the DS also has more "value" games that you can get really cheap.
To be fair, one major reason for this is that the DS has re-energized the 2d platformer, which is one of my favorite style of game (3d platformers suck, and they're plentiful on the PSP).