mobile

I'm in the US and is considering getting a S6. However at&t versions have the ugly logo on the back.....which does not affect t-mobile model. I wonder if I can go buy a full priced t-mobile galaxy S6 (and get it unlocked) and insert my at&t sim card in it. Compared the bands on their websites but the information seems to be partial....Any one could help determine if the t-mobile version will be fully functional on at&t network? Thanks a lot!

Besides, I haven't used t-mobile phones before...Will a fully paid galaxy s6 come to me unlocked? Or do I need to ask them for an unlock code? I see that the tmobile iphones are unlocked by default, but I don't know if this applies to S6..