Ds, hands down. I had a DS for a short while, and I loved every second of it. I'd play on the bus to school, at school, on the bus back home, at home, in my bed at night, then in the morning before I go to school and repeat the cycle all over again. Even the port games like Mario 64 DS offered so much more than the original version that I'd beat it countless times and never get bored of it. WarioWare Touched! was a genius game, utilizing every bit of the touch screen as humanly possible.
I also had a PSP for a short while. Much to my dismay, the only games I could find for it were sports games and racing games, all of which were either straight ports or badly made original games. Sony focused too much on having that system do anything and everything, and not so much on the gaming. I buy handheld gaming systems to play games, and Sony just didn't deliver on that front.
Christmas is coming up soon, and I'm getting a DS Lite. There are so many games on that system I would love to play, whereas the PSP doesn't have one game I want to try on it, save LocoRoco.
That's my two cents, anyway.