Training & Nutrition - Is a Mountain bike just as good as a Road/fitness bike for loosing weight?

I have been road biking/commuting for a year now, out of enjoyment of the sport, but also with the goal of loosing weight. I have recently been wanting to ride taller and more relaxed, So I've been thinking about getting a fitness bike (Specialized Sirrus) to use as a commuter and paved trail rider.Today I took out my Trek 820 mtb, which I have not ridden in about 2 years. I quickly remembered how much I liked riding it (minus my loss of speed). Now my question is... Can i get as much of a cardio workout on it as I would on a road or fitness bike, even though I will be going much slower? It felt great climbing a few hills today, even though it wouldn't shift into the smallest front chain ring. So after a tune up, and putting on urban tires, will it serve me as well as a Fitness Hybrid would for commuting? I still will be expecting to go slower but that can be good for me, since it will force me to leave earlier and will more likely be on time. Thanks for any input.

I would tend to think that it would give you a better workout. Those bikes are heavier, and don't slide as smoothly as a road bike. Mile for mile it will be harder to pedal the mtn bike than it would the road bike. Go for it.

I agree with Bentley. Road bikes are all about efficiency. I'd compare it to road racers having a set of wheels for training and another for racing. The training set being heavier and such.
Just consider the hard-tail your "training bike."

You can get a quality workout with either one. It is all about what you do with the time you have to workout.
Saying one is better than the other is not accurate IMHO.

The only thing that comes to mind, I find it easier to do a longer interval on my cross bike with clipless pedals than on my mountain bike, with flat pedals. Possibly from different muscle utilization? I generally have a slower cadence on the MTB so i mash more and wear out my quads faster.