Not every, or even the largest police forces opt for .40 s&w over 9mm for caliber. Take for instance the NYPD that issues 9mm.
I tend to be skeptical with the "greater stopping power" of the .40 s&w over a 9mm. Considering the quality of ammunition being produced these days in 9mm, the marginal performance increase of the .40 is a negligible value IMO to the additional magazine capacity you get with a 9mm.
But whatever caliber you settle on, shot placement is key. A .22lr in the right place is more deadly than a 9mm, .40, .45 acp, or .44 mag in a glancing shot.