I think from an exercise perspective, a harder workout is a harder workout. So if you ride 20 miles at an average of 15mph on a heavy fat tire or the exact same ride on a road bike then you would work harder and burn more calories on the fat tire. However, chances are you would just ride slower or enjoy it less so ride less often which would be less exercise.

For me, a 90 minute ride on either would probably equate to roughly the same workout, though not the same distance and speed, but I'd enjoy one more (the road bike) and therefore do it more often.