This just reminds me of Jim Cirillo. He talked about how at the beginning of his time on Stakeout squad, the department required officers to carry a .38 Special on duty. But the backup gun was completely open, so Cirillo chose a custom 4" .357 to serve as his "backup" gun. If I recall, Paul Kirchner talked about it in 'Tales of the Stakeout Squad'...
RE: Impact weapons. I would never carry a blackjack or sap, because they are illegal. But if one did carry one, an 8-ounce "cablejack" from Foster would make a fine choice, I think. A little less spring to it, so it hits more like a sap, but large enough it can be swung or held in the fist and snapped out with a quick jab. I keep meaning to investigate saps, something in the way of a small midget, strictly for educational purposes, of course.
-Rob