One, the road bike, is more efficient. You will tend to ride more often, longer, and further. The other, the mountain bike, is less efficient, certainly requires more energy to cover the same ground, so will burn more calories. As pointed out which do you want....
I have seen many figures on the efficiency differences... by +/- more than 20%. That being the case, you could be burning that much more. So given a 35 calorie per mile average... you could burn an additional 7 calories per mile. Given that difference, I would choose the road bike. It efficiency and enjoyment would more than make up for the small difference. I would probably ride less often, at a lower intensity level, for less miles on a mountain bike. Thus, negating the possible difference. That being said, on your proposed test circuit, it is a 70 calorie difference.
My dad used to say, "choose the right tool for the job". Why take a Hummer to race against a Formula One car? They are not the same, you will not ride them the same, you will not ride them identically.....