I always recommend picking a carrier first, then picking a phone (OS). If on TMO the iPhone won't do you much good if for no other reason than slow web on an iPhone is pointless imo. If TMO coverage is solid (Philly should be) then the MyTouch 4G is nice. I would personally also look at the G2 and Nexus S or Nexus One (on Ebay). AT&T is great in your area, so you could always go with an iPhone 4 if she likes it.
If she hasn't tried Windows Phone 7 yet then she should check it out. TMO has the HD7 and the Dell Venue is available to order now and it's a really sweet device. I am using my WP7 phone more than any other right now.