Originally Posted by
woodcraft
Road cycling, IMO, works aerobic capacity more, muscles (especially upper body) not as much, relative to mtn biking,
plus you may get injured less.
This ^^^
Road cycling: aerobic capacity/endurance.
Mtb: anaerobic fitness; quick accelerations/braking; reflexes; bike handling.
That is why almost all mtb xc pros train a lot on the road and on their mountainbikes. That is why some road pros (not all by any means) also ride some mtb, or at least cyclocross.