You'll defnitely find that there will be days where you prefer the trail and days you prefer the road.
I'm a recently converted roadie (as in converted to a roadie), as a really wet summer of not riding the trails can do to a person. As you said, They are very different types of workouts. The trail is fun due to the constant change and alertness, but the road is fun due to the speeds you can push. Having the roadie background and keeping up with it will help because it will keep your legs in much better shape allowing you to only have to worry about building technical ability on the trail. The trail get alot harder if you wear out your legs midride.
The trail limits your speed to an extent right now, but that is mainly due to your technical ability. You will find that you will be able to get faster and faster as you learn to pick your lines and learn how to ride everything. Your mtn bike is not a delicate flower like your roadie is. Your wheels can take abuse and your fork helps even more so.