What are you trying to accomplish by switching carriers?
I've been on T-Mobile since September of last year, and the service meets MY exact needs. For my service, I want a high data allotment, don't use a lot of voice minutes, don't want a contract, only need occasional tethering, and don't care about the rural/outlying coverage. So, I went with T-Mobile's $30 prepaid plan (100 min., 5 GB 4G/LTE data) and paid for my iPhone up front in full. Worked out great so far, given MY specific use case.
People are going to T-Mobile because of the transparency of their pricing, lack of data overage charges, international roaming, ease of transition, and rapid improvements to their service quality in the areas they already serve. They were the first carrier to completely decouple the device and service charges, and that was something that I'd been waiting for. Even now, the pricing for the competing carrier plans remains confusing by comparison.
The main issue with T-Mobile has been, and will remain for the near-future until they ramp up their 700 MHz service, their network coverage. Within major metro areas, T-Mobile's service is generally good and the LTE transition occurred very quickly. If you look at crowdsourced coverage maps, T-Mobile's network performance is now very much comparable to AT&T in these metro locations. When you go into rural and outlying areas, T-Mobile's coverage has major gaps. This is where AT&T has a clear advantage.
As others have suggested, try the T-Mobile test drive. That way you can see if the coverage and features like wi-fi calling work for you.