If you were a cop, how would you respond to this incident?

The difference in performance between the 9mm and 40 cal are not nearly as big as most think. In modern expanding ammunition, the slightly more powerful 40 cal is not worth the added recoil, in my opinion. And to boot, some of the 9mm+p have more ft lbs of energy than most 40 cal loads. Shot placement is by far the biggest factor on stopping any aggressor. Thus when all things considered the potential benefit of the 40 cal is outweighed by the 9mms ability to hold more rounds and having less recoil.(To aid in better follow up shots) For example CorBon 9mm+p 115 gr jhp= 434ft lbs. Winchester 40sw 180gr jhp= 408ft lbs. Now there are loads out there such as CorBons 40sw 135gr jhp that get in the area of 480ft lbs. But they are some what rare. This is not a dig at Winchester ammunition either, just an example of the 40sw offering in general.