It seems Nokia are gearing up for another hit device with the Lumia 635; its first WP 8.1 device ever. According to Microsoft online the Lumia 635 will be available on both At&t and T-Mobile for very competitive prices, starting at $99 off contract for At&t and $129 off contract for T-Mobile. To put that in perspective the Lumia 520/521 first started selling for $130 off contract and was still an amazing hit. Never mind the fact that the Lumia 635 is frankly a better device, but is also cheaper off the bat.
If Microsoft play their cards right they might have another killer Lumia on their hands, and certainly multiple carrier availability is the first of those cards.
You can pre-order the device on both networks from the Microsoft store at the link below: