I'm going through the class right now to become a Reserve police officer here in Jacksonville. On-duty, the issued primary is a Glock 22. We are allowed to carry a bug, and it can be a Glock 23, a Glock 27, or certain .38/.357 revolvers (still waiting to get a list of which revolvers are approved). I would likely also carry it when off-duty as well in a pocket or ankle holster, either as a primary or a backup to my personal Beretta .380.
Considering that, the G23 is already out - too big for pocket carry. That leaves the G27 or a wheel gun. I figure I'd only need the bug if I'm deep in the shat and the primary (G22/Beretta) jammed. (Yes, Glock fans, even Glocks jam. They're semi-autos. It happens.) Ammo cost doesn't factor in - the agency supplies it. It comes down to capacity/firepower versus reliability:
9+1 x .40 S&W
+ Fast reloads
+ Can use same magazines as the on-duty primary
- Larger; harder to pocket/ankle carry
- Semi-auto => failure to load, failure to extract, failure to eject are all possible
+ Smaller; easier to pocket/ankle carry
+ It's a revolver => not possible to have FTL/FTE issues that prevent firing
- Slower reloads, even with a speed loader
- Different ammo
For anyone about to post "Why not get an LCP/Kel-Tec/etc?", my choices are listed above. Your opinions are valid, but not helpful here since my choices are limited. Thanks!