I was on T-Mobile for 4 years and just moved to Verizon. When you put aside all the gimmicks of free pizza and cheaper gas on Thursdays and really compare apples to apples, Verizon and AT&T are way ahead of the game.
I recently drove across the country and needed solid internet for work, I had one T-Mobile phone and another AT&T device. After having to stop about 30 times for internet in random places in the middle of no-where in Arizona, New Mexico and Texas, I think 29 out of those 30 times T-Mobile had no signal. AT&T on the other-hand was strong and had zero issues. There is nothing like being 50 miles from the closest gas station and still able to watch videos on YouTube (not while driving).
I moved to Verizon instead of AT&T due to the better plans and still great signal. I still have one T-Mobile phone, but signal, internet speed and call quality I notice are much better with Verizon.
I always find it amusing taking a T-Mobile phone out of the country, you get the free roaming which is awesome, but the signal from their roaming partners with are better than T-Mobile in the US.