Until Tmo finishes re-farming its 1900 band to support H+, no iPhone (4s or otherwise) will get speeds faster than EDGE (meaning no UMTS, HSDPA, HSPA+, DC-HSPA+, 3G, 4G or whatever the carriers or anyone else wants to call them).
So adam1991 was correct that one does need to do his research and make sure the device they use can fully support the network they choose to be on. If you want the fastest speeds possible on Tmo you will need to get a device that supports H+ 42mbps on the AWS band.
Moderator yahoogroups forum T-Mobile-US http://groups.yahoo.com/group/T-Mobile-US
Say what you will about the iPhone.
It makes a great cat toy.
After I heard about AT&T giving up.