Hi Saint,
I have both and the WOW factor is bigger with the 3DS, the 3D is really great, if you don't move your head too much or the 3DS while playing. The Vita has a gorgeous screen and great controls, top notch quality.
As for the games, I would say it's the usual, 3DS is more kid-focused, with games like Mario, Zelda, etc. And what I find funny is the fact that everybody was saying a year ago that Nintendo 3DS didn't had worthwhile games. Then Mario came out and BAM everyone said that 3DS had TONS of great games... but in fact there's just a couple of really great games and a bunch of 3D remakes of old games.
PSVita on the other hand has also just a couple of great games but with the tastes of your daugther in games I would say this is the one to get. Assassin's Creed III Liberation, Killzone, Persona 4, Uncharted... plus all the PS1 classic you can buy (they are not all available for the Vita though...).
Don't start any flame war here cause I really like both handhelds but I think Vita would better suit your daughter.