I personally own both, a nortic track elliptical that has arms that move, and I just bought a smooth fitness 9.15 treadmill. I get a really good workout on both. The elliptical is easier on my knees. When I do the treadmill I can't run due to a sore knee so I powerwalk at 4.7 and incline to 10.
Calorie wise my machines tell me that I am burning around 500-600 for an hour on the treadmill and only 400 for an hour on the elliptical, but that could just be my machine and machines are often wrong. I have been on some ellipticals that tell me I am burning 800-900 cals an hour.
I guess I can get just as good of a workout on both. Its nice to switch it up, keeps me from getting bored.