I live in Norcal (Yuba City) and travel at least twice a month to other areas in CA (camping, hitting the beach, the snow, taking the kids to the zoo in SF etc). I've used both AT&T and Tmobile and currently have a straight talk sim for each carrier, as far as phone coverage goes I would say they are almost identical. It's very rare for me to be able to make a call on AT&T and not be able to make the same call on Tmobile in the same place.
Data on the other hand can really suck at times when traveling with Tmobile and getting stuck with edge, but it can also really suck with AT&T everywhere as when they both have good data available Tmobile easily beats AT&T. As an example I live outside of the city limits in whats often referred to as southern Yuba City. Using a Tmobile straight Talk sim I get 2.9Mbs down and 1.4Mbs up. With an AT&T straight talk sim I get 991Kbs down and 364Kbs up. In town it's even worse with AT&T rarely getting over 3Mbs down, Where as I can hit 20Mbs down with the Tmobile sim.
I personally have sims on straight talk from both carriers and just call in to switch sims when I know I'm traveling to an area where one carrier has better coverage then the other.