After a few decades with AT&T, decided to change to T-Mobile. Here's what I learned after 60 days. Note I am a frequent US/Canada traveler with occasional travel to Europe and Israel.
Coverage: AT&T is a bit better. That's because AT&T has a lot of spectrum at 850 and 700 Mhz (Band 12 and others). T-Mobile has alot at 1900 Mhz (Band 2). Very little at 700 Mhz and just now is starting to build out 600 Mhz. The 600-850 Frequencies have better range and goes through building walls better. I've noted that sometime when the phone has 1 bar on it and its on Band 2, when I try to make a call, it switches to Band 12, the bars go from 1 bar to 3 and the call is fine. After the call, back to Band 2 and 1 bar.
And some friends and myself needed the T-mobile microcells for our homes. Opening tickets for coverage is useless, generally. There is even a T-mobile store 2 miles from my house with no signal in the back of the store. They claim they told T-mobile but can't get any help.
On I-10 from Jacksonville through to Texas, with T-mobile you will find some brief dead spots of no data. AT&T is more reliable.
But in the Atlanta airport, for years AT&T has had very, very, very slow data. They told me its my phone problem. Really? I had an iphone and Android and it was both phones? BS. T-mobile wins hands down here.
Cost: T-Mobile wins hands down.
International roaming: No comparison. With T-mobiles international free roaming for data and text, there is no comparison. Even free 2G data is good enough for email, web page lookup etc. And I do have another phone on AT&T, and at times, the T-mobile 2G data was faster than AT&T. Don't worry about the 20c a min. Its worth it. In fact, just use WiFi calling in hotels and restaurants for free. AT&T's answer to that is $10 for international roaming. And it really is $12/day with the universal service fund taxes. My last trip cost me all of $1.04 in roaming due to one 5 minute call on an international network. Saved a bundle.
But T-mobile is sneaky in one respect. Showing the the "bars" of signal strength.
Androids show real signal strength in the ABOUT Screens for Network. What T-mobile does is show 5 bars with signals as low as -99dbm on T-mobile branded phones. That's not a great signal. A -50 dbm is the best ever. AT&T shows 5 bars at -50 to -80dbm. T-mobile goes all the way to -99dbm before at -100dbm they show 4 bars. -110dbm is 3 bars, 2 bars =-121dbm.
So don't ever compare AT&T or Verizon to T-mobile due to "bars". There isn't an official standard and T-mobile shows 5 bars where others show 3 or 4.
And here's where they get really sneaky. Assume you take an unlocked late model phone, say and iphone 7 from AT&T and put a T-mobile SIM in it. If it shows 2 bars you get worried. So the stores tell you the phone isn't right for the T-mobile network and sell you an iphone branded T-mobile. And voila, you see 4 bars or maybe 5 bars. My gosh you say, the phone makes a difference. Nope! It's T-mobile showing more bars with a lot less signal. Smoke and mirrors. (In fairness older phones before iphone 6's may not have all the frequencies but new phones have them).
In summary, you can save a bunch of money with T-mobile but coverage due to the higher frequencies is going to lack. But you will save $. And if you leave the USA, you can't beat T-mobile.