Re: Duration or Intensity?
I agree that you shouldn't put too much trust in BMI as it only relies on weight and height and makes no allowance at all for body composition, age, muscle mass. This same problem applies to all height/weight charts. Measuring body fat % is the most accurate way to assess if you are overweight or not.
Your body will also get used to whatever workout you do if you are doing the same thing every session. An hour at a constant pace is great for cardiovascular fitness but not optimal for fat loss. I suggest also changing machines, speeds, difficulties and durations to keep your body guessing.
I found the elliptical trainer was a lot harder work than the arc machine and definitely more challenging. The elliptical machines at my gym also have an Interval Training program which alternates between high and low resistance. You can alternate between both machines, add the bikes, steppers, even rowing machines for good cardio and fat loss workout variations. Also the more variety of exercises you do, the less chance you have of getting bored.