The number of calories torched per minute on various machines depends mostly on how hard you’re working. Higher-intensity exercise requires more energy, thus burning calories at a higher rate.
Another factor is the nature of the activity; is it legs-only (such as walking, bicycling, or running), arms only (arm bike), or full-body (like swimming, rowing, or using the elliptical)? As you might guess, full-body activities generally burn calories at a higher rate because you’re engaging a greater volume of muscle mass — but, again, it depends on how hard you push yourself; you could run like the wind, or just putt along on the rower.
A person weighing 160 pounds working at a moderate intensity (150 watts) on a recumbent bike will burn about 9 calories/minute. The same person, walking at a moderately brisk, 3.5 mph pace on the treadmill (with no incline), burns about 5 calories/minute. With increased speed and incline, the per-minute calorie burn on the treadmill increases significantly.
The key to long-term fitness success is to find several activities you enjoy. If you love walking, but find exercise bikes tedious, use the treadmill even though it will take more effort to burn the same number of calories. For best results, vary your exercise intensity level, your exercise time, and type from day to day and week to week. Make a habit of trying new activities and fitness classes, and mix up your workout routine to keep it fresh.