The primary reason for police and military going to the 9mm is for increased round capacity. The Amadou Diallo shooting in NYC in '99 is an example of why.
The officers ...identified themselves as NYPD officers and that Diallo ran up the outside steps toward his apartment house doorway at their approach, ignoring their orders to stop and "show his hands". The porch lightbulb was out and Diallo was backlit by the inside vestibule light, showing only a silhouette. Diallo then reached into his jacket and withdrew his wallet. Seeing the suspect holding a small square object, Carroll yelled "Gun!" to alert his colleagues. Believing Diallo had aimed a gun at them at close range, the officers opened fire on Diallo. During the shooting, lead officer McMellon tripped backward off the front stairs, causing the other officers to believe he had been shot.
The four officers fired forty-one shots, hitting Diallo nineteen times.
IIRC, the officers were firing from 5-7ft away, and of the 19 hits, only 3 were "mortal" hits. Though the case got national attention and caused all sorts of grief, it's taught in CQC courses today because statistically it's exactly what you would expect from people in their first gun fight. For those that stay in the business to be in a second gun fight, the statistics change dramatically.
If you haven't been in a firefight before, or haven't had extensive and intensive CQC training (spec-ops or SWAT), the recommendation is to go with more capacity. If you HAVE had the experience or training, go with the stopping power.