Good read.From this article: An Alternate Look at Handgun Stopping Power
TLDR: The author studied 1800 shootings to investigate stopping power of different handgun ammunition calibers. There isn't much difference between the effectiveness of the most common defensive handgun calibers (.38, 9mm, .40, .45).
A couple of brief excerpts:
Given the similar effectiveness, it makes sense for organizations purchasing large quantities of ammunition to use the most affordable option.