They are two different types of springs. The SS springs are a specialized spring for racing, but many of us use them on street/strip cars and some on street cars if that is the look they are going for. They will raise the rear of your car substantially, especially if going from worn out stock springs. The passenger side has an extra leaf due to loading of the suspension to the passenger side while launching, so it raises the passenger side slightly higher when the vehicle is just sitting.
I have never ran the HD springs before as I have always had access to SS springs, but many members recommend them as they do not raise the back of the vehicle as high, but still provide a better ride than worn out stock leaves.
Other members here have also had their stock springs re-arched. I have never done this either.