The overall quality of lead shot is always improved by the addition of plating. The plating process hardens the pellet and this allows for better patterning performance. Plating also improves penetration capabilities of the pellet.
Copper-plated shot is a lighter plating process. It is a big improvement over standard lead shot.
Nickel-plated shot consists of a double-plating process: It is plated first with copper and then with a heavy coating of nickel. Nickel is less porous than copper or lead and it is a superior plating material because of its resulting unparalleled penetration capabilities.