You will burn more absolute calories when "hammering", because you will be utilizing additional energy pathways, both fat and glucose as fuel. But you burn more fat as a fuel at lower intensities.

Put another way, The intensity of the exercise is particularly important in determining the muscle's energy source. High intensity short duration exercise relies primarily on the anaerobic pathway for the production fo adenosine triphosphate (ATP). Only glucose, derived primarily from the breakdown of muscle glycogen can be used as fuel.

Exercise of low to moderate intensity around 60% of aerobic capacity can be fueled almost entirely aerobically.