Treadmill for comfort, elliptical for max calorie burn
The treadmill is for walking or running (duh)....that is the most natural movement you could do, while minimizing risk of repetitive strain injury, which is common for people who use the stair machine a lot (can cause knee problems)
you can burn about 5-7% more calories with an elliptical trainer vs the treadmill, at the same percieved level of exertion. I've proven this with a heart rate monitor. Jogging I can maintain 155 beats per minute for a half hour, thats the fastest I can go without feeling out of breath. on the elliptical I can maintain 165-170 beats per minute without getting out of breath, so I am doing more work by incorporating the upper body movement, yet I don't feel I am working any harder.
That said, I still prefer the treadmill - I can get a "runners high" and go into cruise control on the treadmill, but I've never felt that way on the elliptical.