Yeah, MTB is a crazy good workout because it's basically intervals the entire time if you're going moderately hard. On a road bike, you coast and have downhills where your heart rate drops and are generally doing less work/cardio. On a MTB you're almost always involved with the terrain and doing something, so even on the downhills you're working harder to muscle the bike around through corners and shit. I have a bunch of buddies that completely smoke me on the road, who I shit all over in the woods. I like road cycling too but MTB is where it's at if you ask me. The road bike is just easier to get out on during the work week (and it is a pretty damn good time), but in the woods there are no cars, idiots on cell phones, etc, etc.
Granted, there are trees and rocks and whatnot, but I suppose that's the tradeoff haha.