Well I'm a teen as well here, and I chose my HTC 8X, and paid for it myself, with money I earned, and I love it. I don't care what people say, however, I find that everyone I show it too, including teenage girls, also love it. In certain classes I cannot use my phone, because the girls around me are playing with it lol. They always rearrange my live tiles and change my theme color.
Anyways, i'd say to sit down with her, and let her play with a Windows Phone, see if she enjoys it. Show her some of the cool apps like SongArc, show her the people hub, integrated messaging app, all this cool stuff that comes with the MS ecosystem, then ask what she thinks. If she still wants the iPhone, then get her the iPhone, but I think its important to make sure she knows all her facts, and does her own research before arriving at conclusions, not just wanting an iPhone because everyone else has one.
Edit: had to add something about the cameras, her selfies will be better quality on Windows Phone, because WP cameras are really great. iPhone FFC is only VGA quality lol, when I last checked.