Physical exercise. 8 miles of mountain biking versus 8 miles of dirt biking is like night and day on the body. Your legs are the motor, which is good and bad at the same time. Uphills are very different since you can't just twist that throttle. The feel over flat and down hills is similar and can even be more fun since the mountain bike is so much lighter. But your top speed is way less when you are pedaling for power! If I still had my youth, dirt biking rules, but as a dirt fix substitute and in the interest of health, mountain biking has easly filled my dirt passion. Skill sets definately cross over.
You might enjoy this downhill comparison: