That Xl is a pretty good upgrade over its "little brother."
If GSMarena is to be believed, the 640 XL LTE Dual SIM is a monster of a world phone. It has everything needed to run anywhere in the world including the TD-LTE system in China and other parts of APAC.
Look for that unlocked on places like B&H photo and never worry about where you take the thing.
Yeah... Unfortunately T-Mo seems pretty resigned to just a token WP presence. You know, when I go in their stores it is all pretty Spartan in terms of phone offerings period.