I would not get the AT&T or International version just yet. It is quite likely that you will experience slow speeds due to lack of proper frequency support at this time. Eventually T-Mobile's HSPA will be moved over to the 1900 band, however, this is a transition, not a on/off type of thing. There will still be HSPA on 1700 bands for well over a year from now, probably more like two. I think it will be a mix of 1900/1700 bands for HSPA in a couple of years, with LTE in the 1700 band exclusively. So, if you get the AT&T version now, you should hope that your city or region is one of the first for the transition, and don't plan to have good support while traveling. If you are on T-Mobile, you will have to choose great HSPA support now with no LTE ever (T-Mobile version), or no HSPA support now, and maybe some mixed LTE and HSPA support down the road (AT&T version; no LTE on International version).
I would personally get the T-Mobile version if I were you. LTE is not going away, and there will be plenty of LTE phones on T-Mobile in a couple of years. But HSPA has great speeds now, and will still have decent speeds in a couple of years.