Wife and I road rode our mountain bikes all last year and were doing some 30-40 miles rides on them. Then this year we got a couple hybrids. Now we are spoiled and prefer the hybrids. But sometimes we do ride the mountain bikes for 20-30 miles. It is kind of nice being able to go off the road, and slam over pot holes etc.... Anyway we are spoiled now.
To answer your question : I would say u can get an equal work-out either way. If you concerned about time and speed and that kind of stuff - you are gonna see a noticeable difference between the 2. Our avg ride speed increased immediately by 2-4 mph and it became much easier to travel the same distance.