Pokemon X or Y?

Both are technically the same. Both have a few pokemon the others lack, but she can always trade over the internet for any she lacks. Just get either.

X, because Princess Mononoke legendary pokemon.

Y because Mega Mewtwo Y looks swag

Why not both?

This

Xerneas > Yveltal.

Go with X so you can rock the OP fairy antlers of doom.

Get her Pokemon X because X sounds cooler then disown her if she chooses Chespin.

Y. Since Yveltal and Mega Mewtwo Y look so much cooler than their counterparts. Mega Charizard Y doesn't look as cool as X, but better for battle overall.

Meh, they're the same game with a few different Pokemon you can catch, and a different legendary, I got X, my sister has Y. So really, it doesn't matter unless you can find out if she has any preferences, in which case you could see which game has which exclusive Pokemon.

Y for men
X for women. You know... Women got XX. Men got XY ;)

Y is more suited to special attackers (With mega-mewtwo Y and mega-charizard Y) and a few of the other version exclusives whilst X is more suited to physical attackers. All in all the 'better' pokémon in my opinion are in Y

pokemon x and y differences see this, see what Pokemon you like more, and buy the one suits better for you. Also some things like you only have Tyranitar Mega Evolution stone in X, but Tyranitar is available only in Y, same thing for other Pokemon (see here Version Exclusive Mega Stones), personally I like more the Y.