Well, ok, lower gears signify less resistance, but less distance also. Vice versa for high gears. Generally, cyclists aim for 80-90 pedal strokes per minute. If you are really pedaling for exercise, then you will want to pedal fast, obviously. Using a high resistance/low cadence will work your calves and quads, for sure, but it can also damage your legs bones that can't hold up to that amount of force being applied to them. Cyclists are generally thin, and that is because they do a fairly low gear number (high cadence) which builds slow-twitch muscle fiber, which is great for aerobic (endurance) exercise. So, calories burnt by each? I have no idea. But, I use a somewhat low gear (on my 24-speed commuter, I use a 2-3 or 2-4 , with the first number being the front derailleur and the second number being the rear derailleur.
Looking back, my answer seems kinda jumbled. So, here it is, plain and simple (hopefully) - ride with a fairly low gear number, and work up a hard sweat. Aerobic exercise burns more calories faster and you will definitely feel the burn in your whole body, not just your legs like you would with a high gear number.