It is a no contract phone which is different than a non carrier branded phone. The difference in price is about $100 between the T-Mobile branded version and a non-branded version. There are some folks who know how to flash ROMs on Lumia phones. I am not one of them but if you do enough digging around you might be able to learn to flash a ROM on the T-Mobile Lumia 635 and then get rid of the logo. Good luck.
BTW the phone is currently locked to T-Mobile...you are better off buying AT&T branded phones as they are easy to unlock and then use on a carrier of your choice.