I have both of em, and the DS is so much more fun to play with. So many amazing games, and no end in sight.
I do enjoy my PSP however for UMD movies, when Im at work not doing anything or car trips, etc. New games coming out for it have been really fun too, like Maverick Hunter Megaman X, Metal Gear Solid Portable Ops, Ace Combat X, and Valkyrie Profile. Plus a new re-release of Final Fantasy Tactics.