T-mobile is VERY dependant on where you live and where you are, At&t and Verizon very much less so. That's actually fine if you are not one who travels alot and your area has decent coverage. But as someone who travels I can tell you T-mobile can be very frustrating because it is so spotty. I have each, one, on my tablet and one for cell service. I virtually always have a signal and data with At&t, and very thankful I am not dependent on T-mobile for cell service. Sprint it horrific, genuinely the worst for coverage. Our town used Sprint (actually originally used Nextel) and spend a large sum to change because when it was most needed there was no signal or it would just drop. There's a reason Verizon and At&t do cost more.
Good point about build-out, no doubt T-mobile is better now than just a few years ago and seems to be closer in coverage to the big two.
One other thing, it's too much to get into comparing costs exactly, but I know the rates of each, I don't see how you have the same level of service for $50 less.
T-mobile with their unlimited data also throttles after what you pay for, and mean really throttles, to at or below 2G. Their unlimited voice plans are similar to At&t. For regular voice plans I spend less with At&t than Verizon because of rollover, and because I can call any mobile, not just At&t for no minutes. They also provide a device for your home if you have a less than desirable signal, and it gives you full bars anywhere in an around your house, no extra charge. And At&t has a much bigger array of subsidised phones to choose from. In fact there are several things that you have to consider along with coverage before just the cost.
