Be careful when looking at the Wikipedia table. It shows the Republic Wireless as supporting some GSM bands that those in the RW thread say it doesn't.. and the link to the FCC doc is really the SPRINT model, not RW. I'm not sure what other issues their might be.
If you want one single phone to use on BOTH att and T-Mo, get the GSM Developer Edition, or the GSM UNlocked (comes with T-Mobile SIM) from Moto Maker.
They support the following bands (per Moto web site, which might not be updated to reflect possible changes in more recent firmwares)...
GSM: 850/900/1800/1900 MHz
UMTS/HSPA: 850/900/ AWS/1900/2100 MHz with HSPA+ up to 42 Mbps (B5/B8/B4/B2/B1)
LTE: 700/AWS/1900MHz (B17/B4/B2)
If you have T-Mobile, the HSPA on AWS is important to get H/H+ in areas T-Mobile hasn't yet refarmed their 1900Mhz spectrum. No other X supports H/H+ on AWS that T-mobile uses.
The GSM Unlocked and Dev Edition will work with ATT's GSM/EDGE/GPRS/HSPA and LTE networks as it supports the necessary bands. There are two extra LTE bands the ATT branded X supports, however they were originally only used outside the USA. Reports are that ATT might start using one of them in the USA, but as of yet you wont notice it with the GSM Unlocked or GSM Dev Edition phones on ATT.
If you are going "non-contract" check the prices. If the Moto Maker and the Developer Edition are the same price, consider the Dev Edition if you want root, as unlocking the bootloader on the Moto Maker and Retail versions voids the warranty, where was unlocking the bootloader on the Dev Edition doesn't. Only difference would be the appearance. You can't customize the look of the Dev Edition, it comes as black face, woven white back, silver accents (buttons and ring around camera). But you can get a case to cover it, or use dbrand skins
(I have no affiliation with dbrand, just saw it suggested by others on the forum here, so its worth a shot).