Difference between unlocked iPhone 6 from T-Mobile vs. regular unlocked iPhone 6?
If you look closely at http://www.apple.com/iphone/LTE/, the SIM-Free unlocked models (A1586, A1524) supports ALL of the LTE bands of the T-Mobile unlocked phones (A1549,A1522), PLUS some additional networks.

Here is the exact verbiage on the page (easy to overlook): "Models A1586 and A1524 also support LTE networks listed for models A1549 and A1522." which basically means "same as the T-Mobile phone ... and then some..."

I picked up a 6 and 6plus sim-free earlier this year to be used on our mobile share value AT&amp;T plan in the US; the Apple Store employee essentially pushed me out the door with the SIM-free model. Said it had additional bands, potentially better resale value, etc...

Hope this helps clear it up.