T-mobile, with or without Sprint, has spent $10B on frequency acquisitions this past decade -- more than all other combined (probably 2x over)! They even bought part of Verizon's 700MHz band before Verizon found out T-mobile had outbid Verizon for another part of a 700MHz band from the US FCC (coup).
T-mobile's LTE Band 12 (700MHz) is what got them rural access.
And now T-mobile's new LTE Band 71 (600MHz) is what will totally cause them to dominate.
In 2017, T-mobile overtook Verizon on overall LTE performance.
By 2019-2020, T-mobile will overtake Verizon on overall LTE coverage.
Here's a video of how the 600MHz completes their coverage in 2019-2020.
Now understand that 600MHz and 700MHz are longer wavelength (more reach) but lesser frequency (reduced throughput). In the cities, 1700, 1900 and 2100MHz on Band 2 and 4, as well as AT&T's extended Band 66, are going to be faster LTE.
But even 600-700MHz LTE smacks legacy 3G and even HSPA+ (sometimes marketed as "4G") in reliability and coverage, which means Verizon's lead is really falling behind. There's a reason Verizon started heavy marketing once they lost the "mean LTE throughput" to T-mobile in 2017.
I've had T-mobile since 2005. This was after the 2004 AT&T-Cingular debacle that bricked my $500 phone and $3,000 plan (they finally heeded to drop me from 24 month contract to 12 month contract), and after the 2002 Verizon contract I had reneged on their promise of free nights (not just weekends).
By 2008, I found T-mobile faster in New York City than AT&T and Verizon.
By 2013, I found newer T-mobile HSPA+ (21-42Mbps) faster in most cities than AT&T and Verizon.
T-mobile's coverage was an issue in the Florida Panhandle and portions of Alabama before LTE Band 12 (700MHz). But now with both Band 12, and newer 71 (600MHz), if you have a phone that does both, you're looking at the best, high-speed coverage in the country.
Unlocked, but US retail/warrantied (I never buy gray market or use most distributors that outsource, because of Chinese spyware concerns) Band 71 (600MHz) phones will be hitting later this year. For now, only the Samsung S9 and LG V30+ are really options in a new, unlocked, US retail/warrantied purchase.