T-Mobile is the only carrier actively spending cash to refarm old 2G towers to 4G LTE and turning on 700MHz spectrum for phones that can utilize band 12 (Samsung Note 5, S6 versions, the new iPhones that just came out and all new flagships from here on out.) Basically, by the end of Q1 2016, if you buy a new phone, your coverage will be equal to or better than ATT and Verizon.
My wife has sold her iPhone 6+ (she got tired of it always refreshing webpages when she was halfway down a long page!) for a Note 5 and she has LTE everywhere now (T-Mobile just turned on Band 12 in the Fresno area last week). I'm still rocking my 2014 Moto X and I don't have Band 12, so I still drop to 3G in some spots where her phone is speeding along on LTE.
Gotta love T-Mobile. 2 lines with unlimited data with free calls/texts across all of North America and free texting on any flight that has internet for $110 per month.