I used to have T-mobile, but I switched a year or so ago to Project Fi, which chooses the better of T-Mobile or Sprint infrastructure for your current location. It’s awesome with the huge caveat that it only works on a small set of (Android) phone models. If you’re on T-Mobile and you already have a Fi-compatible device, your service will just straight-up be better and cheaper by switching to Fi.
It has international data at no extra charge everywhere I’ve traveled with it, and (pro tip) you can get LTE performance in some places that would otherwise default to 3G (most of China) by manually selecting the better cell network in Android settings.
Disclosure: I’m a Google employee, but I work on VR and have nothing to do with Fi.