I'm going to hate myself, but I would guess the iPhone would probably be more appropriate as a gift, if you don't already know that a Windows Phone is what she wants.
Typically phone choices are very personal and should be aproached with some idea of what you want to do with the device and what sort of experience you want.
A Windows Phone has more capabilities and is much more customizable for usage patterns and just personality than an iPhone, but the iPhone is dead simple to use, and has an unquestionably user friendly interface.
Unless your GF is an inveterate Windows user, and/or has demonstrated some desire for (or even knowledge of) the Windows Phone OS, I think you would be safer with the iPhone.
P.S. My wife, the MCSA, MCSE, MCT, professional IT engineer in MS systems, loves her iPhone, though she clearly understands its limitations.
Sometimes you are the anteater, sometimes you are the ant.