Running and spinning are both effective for weight loss, but there are some major, very important differences.

"Spinning" is just a modern name for "stationary exercise bike". A few years ago, the name was upgraded by gyms and fitness centers to better market and promote group classes. In recent years, studies have proven exercises where you stand up are generally more effective and beneficial for weight loss than those where you sit. This was mainly in reference to exercise bikes, but also applies to today's equipment for spinning.

If you talk to any cyclist (or someone who regularly spins), they're going to say spinning is better. But remember a bike in any form is a machine that converts and uses energy very efficiently. To match a simple run, you have to maintain a very high speed and high cadence (this is the number of revolutions of the crankset per minute). Otherwise (since you're using a machine), you have to cycle significantly longer than a typical run.

For example, a 175 pound person who runs 25 minutes at 7.5 miles per hour (a moderate 8 minute mile pace), burns 413 calories. However the same person would have to bike or spin at 16 to 19 miles per hour (very fast, racing pace) for 25 minutes just to burn 397 calories. This can be verified using any calorie calculator.

To match running, you have to spin faster and usually spin a lot longer. And to match the cardiovascular benefits of running, you have to pedal fast enough to increase your heart rate and breathing, otherwise (if you pedal too slow or apply too much tension), it will lean more toward an anaerobic exercise. You may have to do other outside exercises to ensure you work other body parts too.

A true cyclist on a real bike uses shoes attached to their pedals. These are known as "clipless pedals". They allow cyclists to maximize their pedaling and use the "full circle" (pulling up and pushing down), while keeping both feet positioned in perfect form. To maximize the benefits of spinning, I would strongly advise using these. Most fitness and athletic shoe stores now offer shoes compatible with spinning, but check with your gym first, to make sure you purchase the correct ones. Typically this is determined by a medal attachment near the mid-sole of the shoe.