Difference between AT&T and tmobile's unlocked iPhone?

What's the difference between an unlocked AT&T iPhone and a tmobile unlocked iPhone? I did some research and I found that current AT&T unlocked iPhones are not compatible with tmobile's hspa+ 42 mbps network. Is that true?
Also, are there any other differences?

There's no difference they are all the same phones. It is only compatible with hspa+ in some areas. Tmobile is rolling out there antenas so the hspa+ can be supported with the iPhones.