I think hard and heavy is the way to go. The newer non-tox stuff like the Remington HD, Hevi-Shot (goose), etc kill even better than the old nickel plated lead did. Kent Impact matrix is about as heavy as lead and about as soft and performs just like lead.
Plated lead really isn't any harder, I tested it too, it does pattern better, and it seemed to pattern better too, but that's more of an opinion than scientific data.