NW rocks, the clear first choice.
For me the alleged difference is the quality of the F experience, but since I've never experienced it on CO, my ignorance is my bliss.
I like NW because they treat me right, I always ride in F, I can do same day standby for free, and they always have a bonus mileage promotion of some sort running. Right now, for example, be sure you sign her up for Mile-A-Palooza if you go with NW so she can get some May Double Mile-A-Palooza points.