Nokia N78

As a phone it will run on either network but if you want 3G data speeds you will need to run it on the AT&T network. The N78 will most likely not be offered by either carrier so you will need to buy the phone unlocked (which is better anyway) from a online retailer like MobileCityOnline. They are going to ask a lot when the phone is first released but if you can wait two months or so you should be able to pick one up for around $400.