Arts,
If you mean "most reliable," the best answer is "experiment." If you mean the most effective defense round, the answers are a bunch of theories and few facts. Some say 32 acp hollow point is too weak to reliably expand in the target, and that you're better off with ball ammo. Others say you should use hollow points, and if they fail to expand they'll function as ball anyway.
I know it's best to practice with the same ammo you keep in the gun when it's in your nightstand. But I wouldn't practice with hot ammo like Fiocci (sp?) or Corbon unless p32's can take its regular use. And I don't believe they can. Little pocket guns tend to be delicate.
Dwight M S