T-Mobile's coverage has gotten significantly better in the last couple years since they bought 700Mhz spectrum from Verizon. Before this, T-Mobile had no low-frequency spectrum. Low-frequency spectrum has much better range. According to wired:

"T-Mobile customers saw an LTE signal 81 percent of the the time in the fourth quarter of last year, just a bit less than what AT&T delivered. Yes, both still lag behind Verizon (87 percent), but it’s a level of saturation that lets them claim nationwide reliability."

http://www.wired.com/2016/02/t-mobiles-finally-good-enough-t...

They're within striking distance these days.