I imported a PSP from America before it was released here, and I played it about twice. It games don't lend themselves to play on the move, they're more like console games, where you'd have to sit down for quite a while to play properly. Stuff like Wipeout and Ridge Racer work okay, but you can't really play Metal Gear Solid or GTA on the move.
DS I've been playing non-stop since I picked it up a couple of weeks back. Elite Beat Agents you can play for 5 minutes, same with Pokemon, but both could be played for hours if you want. Also, it certainly feels a lot more durable than the PSP. That huge screen on the PSP looks like it could crack pretty easily, but with the DS folded, I imagine it'd be hard to break.
I prefer the DS.