Road bike will help no end, you ride hills differently.
On a MTB (unless you're an XC racer), the hill mentality seems to be that you just trudge up them in a spinny gear then enjoy the descent.
On a road bike, the mentality is that hills are there to be climbed!
If you're doing off-road hill stuff concentrate on pacing, staying smooth, line choice etc; don't go screaming off up it only to fail to get over the techy bit cos you're exhausted. You want to try and increase the pace as you climb and go OVER the hill, not stop at the top and admire the view while faffing with suspension and body armour and dropping seatpost etc which is why actual hill training is best done alone or with a friend who has similar aims.
On road hill stuff, go hard or go home!
Actually the best way of working hills in is simply to do a hilly ride rather than hill reps, mostly cos hill reps are soul-destroyingly painful and miserable and really only needed if you're doing full on race training.
Doing such rides as a pair will help no end, it gives you some competition (so long as your mate has a similar mindset and is roughly of similar ability)