These are the versions of the Motorola X:
http://en.wikipedia.org/wiki/Moto_X#Variants
The key thing to look at in this linked table is the UMTS bands column - UMTS is the technical name for the 3G protocol. So UMTS -> "3G" for this conversation.
Basically T-Mobile has two 3G networks - one at 1700MHz (also called "AWS") and another at 1900MHz. The 1900MHz network is compatible with AT&T phones (which are also using 3G on 1900MHz) but the AWS band is not compatible with most AT&T phones . Because only T-Mobile uses 1700MHz in the whole world (with a few neglible exceptions) and T-Mobile is not a large carrier, most international phones also don't support AWS. T-Mobile's has gradually started moving networks from 1700MHz to 1900MHz starting last year, and if you are in an area with 1900MHz T-Mobile, your AT&T Moto X will work just like it was on AT&T on their 3G network.
If, however, you are in an area with mixed 1700MHz 3G at 1900MHz towers - which is still a lot of the USA as I understand it - then your AT&T phone will be confused because it only talks one of the two bands, and may drop you at times or have lousy call quality because the handoff from 1900MHz which you are using, to 1700MHz which you can't use, totally messes with the network and your phone... whereas a standard T-Mobile phone (which uses both 1700 and 1900) wouldn't be. This would particularly a problem in a car where you are driving and one tower might be 1900MHz and then your next tower is 1700MHz and now the phone receives a signal "drop to 1700MHz" and it doesn't know what to do with this and eventually drops to GSM but while your phone is trying to decide what to do, your conversation sounds like garbage or just drops. But even when you aren't in a car, it could be a problem due to "cell breathing" which is a weird effect that I don't totally understand but which is that the 3G signal from a tower changes based on how many users are using 3G on that tower (google 'cell breathing' if you want a better explanation) - so due to cell breathing your towers can change even when you aren't moving.
Here's an unofficial map of T-Mobile's 1900MHz towers.
http://www.airportal.de/
T-Mo also have a 2G network (referred to as "GSM") T-Mobile - this is the older network and it's completely compatible with AT&T. If you disable 3G, your call quality will likely improve because your phone won't be confused any more. But your data speed will stink. 2G data is about 0.1Mb/s where as 3G on T-Mobile is usually around 4Mb/s.
My advice would be to turn off 3G most of the time - your call quality should be fine - and then turn on 3G when you need data. If you are frustrated by all of this, then you'll likely need to swap for a T-Mobile Mototola X phone or wait... T-Mobile plans to complete the migration from 1700MHz to 1900MHz by the end of this year... but they are way behind their original schedule so waiting might not be a great option.
Here's another post - from someone probably more authoritative than I am - about it that says the same thing that I wrote:
https://support.t-mobile.com/thread/44406
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