If you use a Straight Talk TMobile SIM you'll get the same coverage you get now.
If you use a Straight Talk AT&T SIM you'll get the same coverage as anyone there gets with AT&T, but you won't get high speed data with that phone - AND you'll have to either get the unlock code for the phone from TMobile or pay someone for it.
Straight talk uses TMobile, AT&T, Verizon or Sprint towers, depending on which phone you're using, they don't maintain their own network or have their own coverage. They get the same coverage as any phone on the same towers. Talk to neighbors and find out which carrier has the best signal where you need coverage - your house, where you shop, your office, etc. Then go with a phone for that carrier, on Straight Talk. (Unless the carrier has a better deal - it happens sometimes.)
