I use t-mo for one feature you cannot get on at&t (nor Verizon or Sprint): wifi-calling. Mainly it means free calls while overseas, but also in "remote" vacationing etc. areas, where no one is getting signal, if I can get wifi it's the same as having full signal. At my home, signal is weak and wifi-calling covers me. The caveat is, T-mo just switched over to "IMS" from "Kineto" and some people report issues on the G3.

 

If you want to know if your flavor of HSPA is faster in your area I'd recommend visiting a TM store and having them do a speed test on the S3. Keep in mind that (at least IMHO) TM's HSPA coverage won't be as good as Verizon's coverage but if you don't travel much, it won't matter. With that said, I have pretty good HSPA+42 coverage on TM in my area.



But this is only in some areas. In most I average 6Mb-9Mb.