As far as I can tell each company has a region they dominate. Verizon has Florida. Sprint/Nextel has much of the south. I do a ton of traveling and have tried a few carriers. My experience is that AT&T is far from perfect but seems to work near everywhere. I rarely have no service. And this is huge to me.
A drawback is international. Their rates seem to be all over on that. As much as $1.25 per text. A 40 minute phone call in British Columbia cost me $200.00
AT&T does not have unlimited net available, and is more money than the others by 15-30%. But, I'm pretty happy all in all. I'm not sure about coverage in the exact region you are asking though.