Most likely the girl. Usually I pick the boy, but I don't plan on using my real name this time, I want to try something different, and the female protagonist looks much better (not in an attractive way >_>).
I'm a guy, but I almost always play as the girls and XY won't change that. There's nothing really wrong with the guy's design, but he just looks a bit dull and not very welcoming. The girl on the other hand looks vibrant and outgoing.
But I don't mind. I never play "myself" in these kind of games anyway, so gender doesn't matter to me.