9mm is the cheapest, and law enforcement ammunition is being assessed primarily by shooting blocks of gel. The ammunition manufacturers have designed their ammunition to perform on the gel test, and the latest ammunition designs don't show any great differences between calibers.
So according to the gel tests, the latest 9mm ammunition is basically the same as atomic bomb grade 10mm or 357 magnum. (ask any handgun hunter if that is true).
A friend of mine is a retired cop/detective.
He tells stories about all the stuff he has seen 9mm bounce off of.