Where to start?

Bullet technology. The advances made in bullet design/construction apply to all calibres so this is basically a moot point. All the same.

45 cal is generally to big in hand grip and recoil is to much for a lot of LEO. This alone counts the 45 out. Unfortunately.

Most LEO have gone away from +P loads. In light weight pistols like the Glock etc these cause a torque twist up and to the right and took longer to get back on target then the 40 cals with a lower velocity that just came straight back. This is no longer a point as they use standard velocity 9mm ammo.

One shot stop myth. Well no it is not a myth, it does happen but very infrequently. Those using a 40 cal have it more often then those using a 9mm, head shots not coming into this.

The FBI summary says that Drs cant pick the difference between 9mmand 45 cal pistol primary wound channels?????? Yep I question that. With ball ammo yes I can see it, with modern expanding ammo where the petals tend to cut not push aside flesh. I think the varsity of there statement is highly questionable. I wonder what the actual wording and parameters of the investigation in to this aspect were. ie is the statistician making his/her own rules for interpretation of data.

Leathality. Plain and simple, the bigger the hole the more blood is let out for a given time the quicker the incapacitation.

Hits on target. Well this comes down to training and using live rounds is a very poor way to train for this once marksman ship has been achieved. LEO's need to be trained with simmunition/airsoft etc. They need to be put through scenario training where they shoot at people and are put under some form of psychological pressure both tactically and pulling the trigger on a human aspect. This more than range time results in far higher hit and accurate hit rates. Cost well the cost of 9mm and 40cal simmunition is not that great and you do not use large amounts of this ammo when training as spray and pay gets you a figurative kick up the bum.

With decreasing number of offenders being drug or psychologically affected LEO need the ammo that has the greatest affect whilst being controllable 9mm or 40 cal fits the bill. + they need the correct training.