For the purpose of prepaid, is there any difference in :
1. buying a used phone that is unlocked and compatible with Tmobile radios
2. buying a used Tmobile branded phone?
Could I buy a phone under the 2 options listed above and just swap my current sim card (mytouch 3G) without any issues?
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).