I'd remind folks that it IS Colorado, and there have been plenty of Mountain lion issues over the years. For example, the guy jogging down near the Springs who was attacked, killed, and partially eaten by one. So all this poo-pooing of bears and coyotes and emphasis on the 2 legged predators isn't entirely accurate. And I wouldn't want to wind up face to fang with one of those carrying a .22. And GilpinGuy is right, a pissed off Moose isn't something I'd want to be near either.
My hunting sidearm was always my S&W 686 .357 and I wouldn't feel undergunned now carrying one in the mountains. A .44 Mag would certainly work, just be a bit bigger. Either one is relatively easy to find ammo for, too. A .41 Mag would work well, if you happen across one like one of the Taurus Tracker. My new general purpose handgun is my Glock 20 10mm, and I carry it as my hunting sidearm with the right loads.
If some folks are happy carrying a 9mm or .40, that's certainly up to them. I carry one when I travel, but not when I'm up in the hills. I wouldn't want to be facing a hungry mountain lion with one.