I wouldn't commute on a bike without "commuter add-ons" - and certainly not on a mountain bike. I really prefer my hybrid over a road bike, but given only the choices "road bike" and "mountain bike" I'd choose road.

My ideal commuter would be 700C wheels with clearance for 35mm tires and full fenders, no suspension (maybe a carbon fork and/or sprung saddle at most), racks, perhaps IGH though that limits frame choice, lights and a Brooks saddle.