I agree that the first question that needs to be answered is how are you determining the number of calories you're burning? If you're just relying on the machine readout, then you really can't use that as any kind of guideline.
The only thing that affects the number of calories you burn *while exercising* is how high your heart rate goes. The higher your heart rate, the more calories you will burn.
However, the *while exercising* part is important. Because although you may get your heart rate up higher while working on the elliptical, the resistance of the stair stepper may be building more muscle that remains metabolically active long after you get off the machine.
There are a lot of variables here ... and like I said, it all depends to start with, on how you're calculating your calories.
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