I can tell you this. I ran into my exson in law at the range and he had a brand new PX4 in .40 caliber. That is one fine shooting gun. I would say the choice is yours about 9mm or .40s&w. Your going to hear a lot of negitive about the .40, but it's all about they, cost more to shoot. So far as shooting, there's not a big difference between the two calibers.