Not all pedal strokes are created equal. Spinning with little or no resistance at a high speed is not the same as riding with more resistance at a lower speed. Both methodologies are part of a comprehensive training program for cycling, but they accomplish different goals. Pedaling at a high cadence with little resistance, you are training your neuromuscular system; the pattern and speed of muscle contractions. Pedaling at a higher resistance, you are training muscle fiber recruitment and force production. In other words, they are basically two different workouts.
When it comes to calories burned per minute, you can achieve equal numbers using either method. However, you'll be able to sustain the effort longer using a high-cadence, lower resistance technique compared to a low-cadence, high-resistance one. This is related to the way muscles fatigue. When you place a higher load on your legs (greater resistance), they will fatigue quickly and you may only be able to maintain the necessary intensity for a few minutes. In contrast, high-cadence, lower-resistance cycling requires less muscular force per pedal stroke, which helps to keep fatigue at bay. That's not to say that pedaling with a high cadence isn't tiring or strenuous. It is, but more of the stress is on your aerobic system. This is why you'll see your heart rate and breathing rate increase more dramatically with high cadence cycling.
Instead of comparing the two techniques in terms of calories burned, I'd rather people look at what purposes they serve for you. High cadence (100 rpm and higher) cycling is great for developing a nice pedal stroke and improving the strength of your aerobic engine. Low cadence (50-60 rpm) cycling, against a big resistance, is great for developing muscular endurance and cycling-specific power. Somewhere in the middle, about 75-80 rpm against a moderate resistance, is perfect for building endurance; it's a difficult enough workout to make your body adapt and grow stronger, but it's easy enough that you can maintain the intensity for 20, 30, 45, or even 60 minutes.
And here's a quick tip: Whether you're looking to develop fitness or lose weight on a bicycle, going longer at a given intensity is often better than going harder for a short period. Riding at an intensity you can sustain for 10-15 minutes, and then repeating that interval three times during one training session (with recovery between each one) is the best way to maximize your fitness gains and weight loss.