the one you want is a tmobile phone. G900T. it will work with ATT LTE, doesn't have a locked bootloader.
the problem is if you walk into a Tmobile store and buy one, they will make you wait 40 days before they give you the unlock code.
you could buy the sim unlock at swiftunlock, but now you are up over $800 for the phone; Tmo wants $690 plus this and that, and tax = $765. Plus $25 to unlock = $790.
$800 was to much for me to pull the trigger.
But you can then have an unlocked, flashable, LTE capable phone.
The last thing you might need would be region away from @Chainfire
if you intend to travel abroad and use local sim cards upon arrival.
This last step is the one I'm waiting on; the region away. not sure if we'll see it. i hate to be a bother and ask.. i think he said he was on vacation for a month or something