One other factor that comes into play is an anti-terrorism focus, where a line officer (or agent) may have to head shoot a suicide bomber, or someone equipped with body armor.

A hit in the brain with a 9mm, or .40 will be about the same, it's just that hit has to be made, and maybe a bit less recoil will assist some shooters in making it.

"Put none but Americans on guard tonight."
-George Washington