Your mtn bike will climb better or I should say "easier" than anything else but they are slower. If your commute will increase in distance by 2-3 miles changing bikes will not make much difference. Allowing a little more time would be needed. A road bike or cyclocross bike is faster and more nimble and actually more fun like mentioned above but you lose the utilitarian aspect of the mtn bike as it lends itself to be a multiuse all terrain commuter. You can also put commuter bars or offset mtn bike bars on a cyclocross or road bike if you choose that. I have tried those out and they were almost as comfortable as a mountain bike after that simple change.