The "mm" in 9mm, means "might make it." I remember reading some information a very long time ago regarding the penetration of the 9mm, .40 cal and the .45 ACP. If I recall correctly, the 9mm penetrated the test medium to the minimum depth... only 68% of the time. The .40 cal was in the 80-90% range. The 45 ACP was a constant high 90's%.
Other things to consider would be wound channel generation. If you happen to find yourself in a critical incident and you had a choice of the two calibers, 9mm or 45 ACP... why choose the one caliber that performs minimally 68% of the time? 9mm just does not create the tissue damage a .45 ACP does. I have seen a lot of folks shot numerous times with 9mm rounds. Sure they bled some, but it was no sure thing they would cash in their chips or even stop the fight. The .40 cal was and is much better. The .45 ACP is tops. I have seen more one shot stops than the 9mm could have ever hoped for even in a controlled setting.
I know shot placement is critical for all calibers. If you are talking about handgun rounds, I'll take a .45 ACP for an auto pistol. If you are shooting something like an MP-5, and can just buzz saw through your adversary, that is a different story.
The concerns for over penetration can be over blown at times. Being aware of how your house is laid out is certainly critical, as is knowing what is potentially down range. Inside my own house, all that gets tossed out when it hits the fan. Outside and on the street those things are considered.
You cannot predict what the environment will be if and when something bad happens. Ball ammo has worked fine for me.
My two cents.
