Dungeon. I'm a huge KH fan, but you could easily skip this game like I did.
OR, if what I suspect is correct, and you just want the game to enjoy the game, pick Dungeon anyway. Seems better to me. But, I have not played either game, so, uh, I'd probably need to shut up now. From what I've heard, seen, and watched, though, Dungeon looks to be the better game, gameplay-wise.
Edit: But with many other games to choose out there, many superior to these listed, you might want to take a spin on some of the other DS games to come out, or maybe save your money for something else.
After all, with this kind of holiday season coming up, we'll all be broke before we can finally say,
"I'm bored of Animal Crossing, now."