The biggest diff would be if the unlocked gsm device has the 1700 mhz AWS high speed data channels that a Tmobile branded phone often does. If you are happy with 2G/feature phone level data, any gsm phone on any gsm network will do but if you want high speed data in the 3g/4g level areas, a Tmobile branded device would do better. Generic unlocked gsm devices often have variants and many often have 850/900/1900/2100 mhz 3G or better channels but lack 1700mhz channel. Nokia Symbian 3 phones are some of the few that have both. There are more international carriers that use Att's 3G channels (850/1900 ) then Tmobile's (Wind in Canada is one of the few that does).