Hi, I'm going to relay a little info that I just picked up in my new Muscle & Fitness Hers.
One hour of aerobics (not intensity drills i.e. plyometrics or circuit training with weights) burns 426 calories if you weigh 150lbs...398 if 140 lbs...355 if 125 lbs and so on.
Running depends on weight and how fast you run...i.e. if you run 5.5 mph and weigh 150 lbs you burn 740 calories/hr. 150 lbs at 7 mph you burn 920 calories.
The chart breaks down a myriad of activities into calories burned per hour determined by your weight. It's the April/May 2001 issue, if you're interested.
Personally, I find that unless I am doing intensity drills, plyometrics, kickboxing or something more apt to give me cardiac arrest, that plain old aerobics just don't cut it for me. I found that when I added running (and I'm slow, about 6.5 to 7 mph, I found it easier to lose the pounds. I also find x-c skiing very good (and enjoyable!).
Amy