I alternate my cardio days between the elliptical and the treadmill. And I notice that I burn a lot more calories doing the elliptical than I do on the treadmill; however, I really feel like I'm getting a harder workout on the treadmill.
I do roughly the same amount of time on either machine. On the elliptical, I just run it straight through; adjusting speed/resistance to whatever "hill" I'm on. Usually, the higher the hill, the more I up the resistance, and the slower I go, and the lower the hill, the lower the resistance, the faster I go. My heart rate usually hovers in the 150s. This usually burns ~350 calories.
On the treadmill, I alternate between brisk walking and jogging. This last time I went, I did 5 minutes walking/5 minutes jogging for 30 minutes straight. When I walk, my heart rate is in the 130s or 140s. When I jog, my heart rate shoots up to the 160s, sometimes going into the 170s. I always feel like I'm working harder when I do this workout than when I'm on the elliptical; however, I only burned 210 calories.
If the latter workout seems to be working my body more, then why am I not burning as much (or more) calories than I am when I'm on the elliptical? Seems like I'd burn less on the elliptical and more on the treadmill with the way my body reacts to it.