I haven't played the PSP version, so I can only recommend the DS version.
What people say about the PSP version is that the controls are better, better graphics, but slower load times. The DS version would have more precise controls since you're using a stylus, but your hand can get in the way of what's on the screen. I never had that problem, however. I would say get the PSP version, even though I want to recommend the DS version because I'm a little biased since that's how I first fell in love with the game.
I've been debating picking up the PSP version for quite some time now.