what is the difference between sony ericsson tm717 and t707?

Band IV

According to the specs at gsmarena, the TM717 uses the 1700 (upstream) and 2100 (downstream) Mhz frequencies for HSDPA (3G). The T707 uses only the 2100 Mhz spectrum, so it won't do 3G on the T-Mobile network.

I can't find any other sources to corroborate this information (including on the Sony website), but it seems consistent with a number of other phone types they've produced.