It's more of a proportion, along the lines of what Ericm979 just said. the absolute number of calories burned from fat is likely to be more with the higher intensity. The heart rate is a rough guide to intensity of exertion, but I have learned that for me, the ambient temperature is a major variable. When I run in hotter temperatures or higher humidity the heart rate is way up to about 90% of predicted max, and I don't feel like I'm killing myself.