Originally Posted by
knobster
I agree with Looigi. If you're wanting to use it strictly for exercise, then you want to pick the bike that's going to make you work the most. To maintain the same speed, you'll work more on the mountain bike than the hybrid. If you were using it for transportation, then you'd want the bike that makes you work the least to keep it at speed.
But that depends upon whether you are riding for a consistent level of effort or a consistent level of speed. When I ride for a workout, I push myself to maintain a certain level of effort, regardless of how fast I am going. Being able to go faster for a consistent level of effort is more rewarding and motivating for me so a lighter, faster bike is better in that case. Just a different perspective on this.