Verizon service is known to be the most reliable across the country. In heavily metro areas, you'll usually see stronger ATT service, but as you leave the metro areas, that service degrades quickly. Get to a massive rural area, and ATT frequently has no service. Verizon, on the other hand, has 4G almost everywhere, and where there's no 4G there is usually 3G. Significantly fewer dropped calls, too. At the moment, Verizon's LTE is faster than ATT's as well. (They go back and forth and each company likes to claim that they're always the fastest).
In terms of service, from what I've heard about T-mo, it's a trade up. I've never heard MUCH bad about t-mo, but I've heard that verizon/ATT are better. And I can say first hand that verizon is more reliable than ATT.
..thanks again