From my experience, which is admittedly limted, when it comes to roundballs, shot placement makes a BIG difference. I shot a deer at about 50 yards, hitting it behind the shoulder, and had good expansion and penetration. Another one at roughly the same distance I shot at a quartering-to angle. This ball went through the shoulder blade, then broke into at least 4 pieces. Enough got into the "boiler room" to do the job, but the difference in terminal performance was amazing.
I don't know how much difference there is in hardness of lead balls, but IMO I would opt for a little more penetration than risk fragmentation from a too soft ball.

IM jaybe