I always shoot a pure lead roundball. I want that roundball to engrave to the rifling on the way out. There is no advantage I can see for shooting a hard roundball. If it is penetration your after the pure lead one will do the job just fine.

I want that roundball to penetrate and then when it hits something start to flatten out and bounce around. A hard one will just poke a hole through the animal.

Also when shooting conicals, you want pure lead also. Otherwise you can tend to lead the bore of the rifle up a lot faster...