I live in San Francisco and have T-mobile service, and don't see what you're seeing. I'll hop on up to Bay and Hyde tomorrow to double-check, but in the past I've gotten adequate coverage everywhere aboveground I've been in the city.
The voice calls were cutting out on a Nexus 4 on those streets when walking on the right side of the sidewalk North from Broadway to Bay and walking East on Bay from there.
I also never used ATT so I don't know how their service is.
Basically, the only time I've ever gotten good quality calls from T-mobile was when I was calling the emergency number from an area I normally have no service in.
Also, you are aware that ATT and TMO have a roaming agreement? http://www.theverge.com/2011/12/20/2649023/t-mobile-to-get-s...