minus the age I'd go with the PSP hands down, BUT since were talking a 6 y/o DS would win my vote... BUT if you already picked up a Wii skip the handheld and pick up some more swag for your Wii!
lots of goodies for that
I've had mine since they first came out, LOVE IT! (Granted I hardly get to play it lol, but the rest of the house loves it too)
-Loki-