I have used both the treadmill and the elliptical. But for me the elliptical wins hands down. I think this is because-
1. As stated in posts above, the elliptical is easy on the joints.
2. The elliptical is powered by the user himself.
The machine I use is the Life Fitness range, a rear-drive machinery with different levels of resistance settings. As such, I worked up to level 18 (out of 25) which is the fat-burn/cardio level.
By typing my exact weight in pounds, in my case 210 lbs, the console reads my calorie burn rate very accurately. In a single 50-minute workout (including 5-min cool-down) I can burn off over 1,080 calories, as I did this morning.
Instead of the treadmill, I find running much more exhilarating. I train with a partner, wherein we can talk about business and social matters (by the way, he's straight), making the run less tedious. In the summer we pass through the woods, in the winter, when it's dark, we stay on well-lit paths. Like with the elliptical, we have added distance, but unfortunately, we have no way of gauging distance or calorie burn, but on duration only, when we can run non-stop for almost an hour.