I have the med. duty coils in the rear, but looked into purchasing the heavy duty. When I spoke to the rep. in Washington, he told me the heavy duty was a constant load spring. They recommend them for vehicles that carry a heavy load all of the time... tools, work vehicles, etc.
I was told the med. duty are a variable rate coil. I've never had an issue with the rear end sagging, even with two weeks of water, gas, etc., and it rides very smoothly when unladen as well.
The valving on the OME shocks and struts have show excellent performance in conditions varying from snow to high speed desert tracks.
Good Luck