Let me start by saying I work for a large agency. I'm also not an expert or in a position of training others or making these sort of decisions. We carry HK 40 pistols, and have access to our choice of HK UMP also in 40o, AR M4s of various sorts (some select some semi only) or some sort of 870, most of which are 14" models. We are in the process of phasing out the UMP, and in general I'm extremely underwhelmed by HK's service to us. The last time we had two UMPs out of service, it took well over a year to get parts.
Also, although I admit my experience with piston uppers is limited, I can't imagine a law enforcement need for a piston upper which necessarily equates to choosing some small increment of severe useage reliability (in conditions which an LEO rifle won't see) over a fairly dramatic decrease in accuracy. Given that I choose a rifle for the express purpose of having the option of making a headshot if need be, I would never consider going from a 1-1.5 inch rifle to a 3-4 inch rifle. If your agency uses suppressors (we don't except for special operations), then maybe the calculus changes.
If it were up to me, I'd carry lightweight 16" carbines with a mix of optics.