Personally I think Xbox overall, but PS4 seems to have a better interface. They really are pretty much the same in my opinion like not much difference between them both, the biggest being the controller.
I've had a PS4 for about a year and I love it! COD is great and I'm sure she'll also love Fortnite. I honestly think that you have more control with the PS4 controller than an Xbox - used one for years before my PS4.