For the US, it's very important to check out the coverage in the EXACT places you will need it. For instance, in Philadelphia, my mother gets an excellent T-Mobile signal, while my friend just 15 blocks away got a poor one until recently (it was much better on my recent visit than it had been in the past, as T-Mobile is beefing up their 4G LTE network).
Also, if you're buying the phone from T-Mobile, their phones have some extra frequencies that are used in rural areas in the US (specifically, 4G LTE bands 66 and 71). These bands are not present on most other phones, so if you're planning to buy an unlocked phone, it may not work as well in rural areas as a T-Mobile phone (in urban areas, T-Mobile mostly uses the frequencies with lower numbers, like 2, 4, 5 and 12; as long as your unlocked phone has these frequencies, it should work fine, as mine does).
T-Mobile works with Wi-Fi calling, so as long as you are using a phone that supports this (my new one doesn't, but all those bought from T-Mobile do), cellular coverage isn't such an issue in your home.
Summary: talk to people who have T-Mobile, who use their phone where you specifically want to use it, before switching.