Its hard to break up...but after all these decades, I'm leaving AT&T for T-mobile. Aside from a great data plan, the T-mobile ONE plan is really a great financial deal. Aside from a cheaper monthly rate, its advanced data network is faster and the coverage is better. You also get free unlimited calling, data, and text in Canada and Mexico. Then there is the unlimited data and text in 140+ countries -- including the land of smiles. The details of the plan are here: https://explore.t-mobile.com/t-mobile-one. I don't plan to use the phone a lot in the Land of Smiles because I have pretty decent service already with what I have and its cheap. But there is always the option to have it as a second simcard option on my current phone because it supports two cards. Then there are some plans to visit some other countries in the future, including a trip to the holy land in August or September. Traveling to places within SE Asia (excluding Laos and Vietnam) is also covered as it will give me an option to not buy a local simcard to keep in touch via data and messaging.
Another reason for switching is that I have been having a big headache getting my Samsung Gear 3 Frontier to work on the AT&T network. My current plan will not allow me to add the watch without having to create a another new plan for $40 more, and it has data limits. With T-mobile, I can use their Digits feature and have the same number on my watch, tablet, or other connectable devices. It also can tie all of your other numbers into one device if you need that as well. I simply call it the 'many to one' or the 'one to many' number feature.
For those concerned about having to change numbers, there is something called 'number portability' which is a government regulation that allows one to bring their old number to a new carrier. In other words, you can keep your existing number.
As for coverage, check out the following website or click on the image above as it provides user data on the areas covered and how strong the signals are in that area for all the major carriers in the US: https://opensignal.com/network-coverage-maps/
This is the second service that I've pulled from my former company. Sadly, it painful to see how they have fallen behind in technology and service offerings. If that ever changes and they get aggressive and innovative again, I will gladly move back, but for now it hello T-mobile.