A few things.
1. The only HTC One worth getting is the Tmobile version, neither the AT&T or Playstore version have the 1700 HSPA band, though it will have the 1700 for LTE.
And the tmo version will not need to be unlocked, it will be running on tmo towers.
2. Tmo HSPA is faster then metro LTE and tmo LTE is faster yet
However it will depend on where you live some areas the tmo coverage is not as good and so it would not be worthwhile to switch.