Different machines, different calories??

Hi,
Since starting work again, I have had less time to go to the gym so I purchased an elliptical strider, here is a link:
http://www.machine-rowing.net/wp-content/uploads/2011/01/elliptical-strider3.jpg

As I can't afford a proper HRM, I am easily confused as to how many calories I'm burning as the machines estimations seem way off. The action is easier than a crosstrainer/elliptical trainer and I find I can keep at it up to 3 times longer than on the crosstrainers at the gym.

I feel I'm cheating myself by putting down elliptical trainer into the MFP calorie counter as it doesn't seem to take half as much effort as the ones at the gym, obviously due to the different actions of these machines:
http://www.tekfitness.co.uk/ekmps/shops/tekfitnessltd/images/life-fitness-95xi-cpo-cross-trainer-1256-p.jpg

Do I just class it as walking quickly? Does anyone have any suggestions?
Thanks,
Vix