For those of us who have crappy coverage with TMO, Verizon themselves offer a much better alternative to their postpaid service, called Visible. It's pretty hilarious how much better of an experience it is, and you are on the same network.
Coverage is very specific to your situation. I've had basically no coverage on Verizon in offices in the Bay Area where T-mobile worked fine while colleagues could only get Verizon at home.
I haven't had any issues with tmobile coverage (that wasn't also a problem with verizon) in well over a decade now. Hell it even worked well in the dense hilly jungles of burundi. Verizon customer service was so bad before I switched I swore them off for life....
The single place I noticed verizon gets coverage and tmobile doesn't is three levels underground in a concrete parking garage.