I'll say that, on average for police use there's no effective difference between 9mm and .40S&W.
There'll be more variation between different bullet weights and loadings.
I mean, look at the trade offs:
9mm gives you more rounds, 16 vs 12*
9mm has less recoil
.40S&W has a vaguely defined additional amount of stopping power.
.40S&W maybe has a reduced chance of over penetration.
9mm costs less for range time
Personally, I'd say 'let the officers try out a number of different weapons and keep the one they like/do the best with'. Heck, have them purchase it themselves if they don't want the 'base' model.
Personally, I carry my 9mm. I shoot more accurately with it than I do my .40 or .45. With the AWB gone I have 16 round magazines for it. It works. The magazine with it is tested regularly, and has never failed. Every failure of the gun to operate 100% reliability was traced to ammunition combined with environment(don't shoot blazer at -30), or a faulty mag(marked and set aside for possible repair, though at this point I'm likely to simply toss it, 100% new magazines are cheap again). The accuracy deal is the biggest deciding factor.
Yes, caliber and ammunition makes a difference, but once you get around .380 and up, a center of chest wound is generally disabling. A .32, for example, has a significant chance of not stopping the target with a COM hit compared to a 9mm, .40, or .45.
Still, like stated, I'd be looking into how many times the police have fired, hit their target COM, and still not disabled it. If that's significant, then I'd look into trading up. Even then, maybe a change in bullet selection, or switching to a +P load might solve the problem. If the main problem is that the police seem incapable of hitting COM, it's time for more or different training over new firearms. Unless it's found that the firearms are inherently inaccurate, of course.
*On this point you can lay the blame for criminals carrying larger calibers on the AWB. During which people tended to buy larger calibers due to the magazine size restriction. Many .40S&W pistols were sold in favor of the 9mm when the 'advantage' of a 15-16 round standard magazine was lost.