Originally Posted by
Kapusta
Assuming the same exertion for the same time, it would be a wash.
It not that simple. The problem your analysis is assuming the same exertion. A mountain bike with knobby tires is harder to push along over a given distance than a road bike. Aerodynamics and rolling resistance are higher for the mountain bike than the road bike and thus mean that the mountain bike takes more energy to move over the same distance at the same speed as a road bike. You could increase the speed and effort on the road bike but then the exertion isn’t the same. And, if you increase the speed and effort on the mountain bike, it takes more energy to move it.
What it really comes down to is having the bike that is the most subjective fun for you which in turn means you are more likely to ride more, which actually does make you more fit.
I don’t disagree with this.