Okay, yeah, you'll burn more calories from fat at a lower heart rate BUT you'll burn more calories TOTAL at higher intensities...for example, let's say you burn 200 calories doing 30 minutes of lower intensity cardio...average fat burn is about 60% at low intensity, so you'll burn about 120 calories from fat...now, say you do that 30 minutes at a higher intensity and burn 400 calorites...sure, you'll only burn about 35% of your calories from fat, but even then, it's 140 calories from fat, plus double the total calories...
I guess it really depends on what your long-term goal is...bulk up or lean out?? Do your cardio AFTER your weights...preferrably an 8-hour window between the two sessions, if possible...