I'm actually going to jump to T-mobile in about 18 months. From articles I'm reading they have been dumping large amounts of money into their networks, actually T-mobile coverage is better than AT&T in this area.

How I rank providers by coverage:
-Verizon
-AT&T
-T-mobile
-Everyone else
-Sprint

Price to coverage value:
-T-mobile
-At&T
-Verizon
-Sprint