the mountain bike with street tires wouldn't be a bad idea. Commuting on knobbies, for me, would be horrible. As you're riding to work you want as few things possible to dread, and knobbies on pavement are certainly dreadful... hehe...
but if I had the choice I'd go with the road bike. Click the link below, the Raleigh Technium - thats the bike I ride to work. Not great, but it gets the job done.