I think you should get out and find a hiking trail on a mountain near you.
I am concerned with making my knees last as long as I can and I find that doing this works great and burns tons of cals.
I go hard on the incline, not running but power walk up it, and then tighten your quads up and hit the breaks and go easy on the decsent, I watch people run down the trails and it just makes me cringe to think about the impact they are putting on their knees.
I think the Stairmaster beats the elliptical in the gym, but I find that everyone that only does gym cardio cant keep up with me when we go for a ride on my Mt Bikes or go hike a trail. Real world cardio vs Machine cardio has a great amount of difference.