Given the typical attitude of teenage girls to cycling, I'd be inclined to give her what she wants, on the basis she's more likely to ride it.
That said, rack mounting and room for wider tyres is always a good thing...
This, so much this.
Get her the best thing you can. The more fun it is to ride, the more likely it will be ridden. I am fed up with seeing people give up on cycling because they have started on a crap bike. I see it so often, guy bys gf a bike so she can come on rides, but he's not sure if she's going to enjoy it, so he gets her a cheap bike, so they go out, him on the lightweight di2 ultegra wonderbike, her on the 200 quid thing from decathlon. It doesn't fit great, it's hard to ride, and just not fun. She doesn't enjoy it, and thus it ends up in the shed, and eventually gets sold. I met one woman on a group ride a year or so ago "I never used to enjoy cycling until I tried a decent bike"
A bike that is fun and easy to ride will get ridden more. Don't skimp. If she grows out of it in a year, fine, sell it on, buy another bigger one.
If you can factor in that it can take mudguards and a rack, maybe she'll end up taking it to uni etc... too. But you have an opportunity here to either kill, or cultivate a life long love of cycling. One shit bike is all it needs to put someone off.
Oh, and get a really good lock, and make sure it's easy for her to use and take with her. You don't want it to get stolen.
J