Don't forget to the other games they announced for the next-gen handhelds. There was Puyo Fever and a mysterious Project S (Sonic maybe? Shinobi?) announced for PSP. There was also a Project Rub announced for DS, which as a rather sill name at the moment, and will probably involve the touch screen in some key way.
Right now, I'm leaning more towards the DS than the PSP. The deciding factors are the backwards compatibility, the longer (estimated) battery life, and a significantly lower (estimated) price. And now they have Sonic.