Basically an elliptical is just another cardiovascular exercise similar to treadmill. And what you're going to be doing is working on your cardiovascular efficiency. So you're going to be burning calories and working on your cardiovascular system at the same time.
Now I believe that any cardiovascular, or any cardio exercise period, you should be most intense. All right. So that means burning more calories per minute is going to be the best use of your time, instead of going low intensity.
A lot of time they talk about if you want to burn body fat, to stay at 60% of your maximum heart rate. Now that's all fine and good, but if your goal is other than just body fat, or even body fat alone, your goal should be to burn the most amount of calories in the shortest amount of time. That's what I think is most important.