Huhuhuhuhhhh.. Hey Beavis... he said....
*snirks*
Only slightly better than 3G on the current iPhones. The AWS band is 1700MHz, PCS is 1900MHz. Not much of a difference.
It's the 850MHz CLR band that has excellent penetration, and T-Mobile doesn't have this band. AT&T does, but not in all areas. I've found NYC coverage to be hit-or-miss. Lots of tall buildings + high population density + horrible multipath due to reflections off said buildings == nightmares for RF engineers.
Edti: I should add that AT&T and Verizon have repeaters in many buildings. For example, when you walk into Penn Station with your AT&T iPhone 5, you'll notice that you go from having LTE to five perfect bars of "4G" UMTS coverage. The reason? AT&T repeaters throughout the station. This is universally case when you see full signal "4G" coverage deep in a building on AT&T. These repeaters don't support LTE but you should be thankful as they're the only reason you have coverage in there in the first place!
With the new AWS tmobile iPhone the building penetration is 10 times better. Tmobile has a 6 or 7 year roaming deal with AT&T on 850mhz. This was well worth the wait.