It's two different sports altogether. I have done both. Both "can" be equally as demanding, but to me at least, MtnBiking has better scenery, and is slower & more relaxing mentally too. Bursts of energy are more often offroad, while long endurance, and stamina are most needed by road riders.
I honestly don't feel that breathing in all the exhaust emissions is very healthy either. Sort of the "More Harm than Good" saying....
Our mountain air quality here in Western, NC isn't very good eiter though, thanks to all you commuters out west. Maybe the earth will start spinning the other direction someday, huh?