I live in the suburbs of Los Angeles. T-Mobile's always the fastest LTE for me...IF I am outdoors. As soon as I go into any kind of concrete structure, like my office building, the mall, a large grocery store, etc, I don't even get a signal from T-Mobile. Total dead zone. Meanwhile, VZW's almost always pushing a strong, usable 4G signal everywhere I go from Castaic to Long Beach.
T-Mobile's great if you want to save a few dollars per month and don't need good reliable service inside of buildings, but if getting a strong signal all the time is more important to you than getting a super fast signal while standing outside underneath a tower, I'd stick with VZW.
VZW admitted late last year that they've let their backbone stagnate as customer volume grew, and we're taking steps to improve it.
I use my office building in Valencia, CA as a guide for me:
VZW pushes 4G into most of the building, and 3G all the way into the central elevator core.
AT&T can push 4G about 10 feet into the building, 3G a little further.
Sprint can push 3G into most of the building (I have a Ting SIM).
T-Mobile can push EDGE about 4' into the building, dead zone anywhere beyond that.