The DS had a much stronger overall library, including a metric ton of Dragon Quest, Pokemon, and countless others, so that's what I'm going with. The downside to the DS was that I didn't like how touch controls were made a focal point of the two Zelda games. They're still good games, but I'd have killed for conventional controls. The ability to draw a path for the boomerang was pretty fun though. I also played a lot of Mario Kart DS, and Infinite Space, Platinum's second game, was a rare gem.
The PSP was pretty good, too. My wife got me mine after my DS got stolen. Had a solid library of RPGs like Crisis Core, Valkyria Chronicles 2, and especially FF Tactics: War of the Lions was probably my favorite PSP game overall. The clearer translation and redone cut scenes made a world of difference though the load times were a little rough.