I reckon a ‘full-on’ XC mountain biker is going to be a fitter rider than a ‘full-on’ road cyclist. You only have to listen to the incredulous comments from ex-roadies when commentating on XC mtb races on the TV, about how high and sustained the riders heart rates are, etc.
some XC racers have managed to transition to road riding quite successfully- Jean Christophe Peraud podiumed last year in the TdeF – but it’s the sustained effort over long periods that roadies do over and above mtb racers. A few pundits last year mentioned that Peraud was in such good shape for his age because he hadn’t spent years riding himself into the ground in the way pro roadies do, and he was likely to have a longer career because of it.
The average recreational MTBer will be more technically competent but less aerobically fit than the average recreational roadie. Most pros in either field do both as complementary training (along with the F1 and MotoGP guys), and the likes of Marianne Vos and Ferrand-Prevot seem equally happy and successful on fat, thin, knobbly or slick tyres.
I have gone from the back of my circle of MTB mates to the front on climbs since I took up road riding, and luckily haven’t lost the bike handling skills (touches wood and doesn’t include getting taken out by someone else on Roubaix Pave…).