I commute on both and of course the road bike is faster but not by much. It feels like its rolling easier but I arrive at work at the very same time. The Mountain bike with slicks is about 3 lbs heavier than my steel road bike but its more comfortable and I can use it for off road jaunts to change things up a little, so I arrive in pretty good shape even though its slightly less efficient. I can put studded tires on it for winter and then it gets really slow but at least I can go safely. I enjoy the road bike better but only because it feels more nimble. In reality the difference isn't as great as you have experienced unless it was a 14 lb carbon fiber racer.