I was in the same position as you. I was under at&t using a Galaxy S4, my contract was up in May, and the GS4 had screen cracks and some other device issues. I thought going with the GS6, but went with the LG G4 instead for its microSD slot and removable battery. I wanted to switch to tmobile for a few reasons, mainly due to international travel:
1. At&t charges you for exceeding your data limit, tmobile throttles your data speed instead.
2. At&t offers a limited amount of international roaming data packages at set prices. Tmobile give you unlimited data (in many countries, but not all) at no extra charge, the catch is it's limited to 2G speeds, which should be enough for email and messaging.
3. Tmobile allows for its calls and texts over wifi, at&t does not. If I'm outside the country, I can text and make calls to anyone in the US at no addtional charge when using wifi calling, I can't do so with an at&t account. Wifi calling is also useful in case you need to make calls or texts in places with wifi but no cell reception, or places where cell reception is not allowed due to interference.
4. Tmobile is generally cheaper, although this wasn't a big decision factor for me.
Having said the above, I would check your reception and coverage in your area before making the switch. In terms of coverage for my area, it's about the same between the two networks (I live in Northern VA)