The reality is, as has been determined over and over ad nauseum by the militaries of the world, the pistol is largely irrelevant in military/paramilitary combat. That having been said, as the man pointed out, that doesn’t mean it isn’t important to the individual soldier who suddenly needs a sidearm and happens to have one on his hip. I carried a pistol in the field from the first moment I was afforded the opportunity, and never regretted the extra weight.
9mm, .45ACP, .40S&W; the caliber is ultimately irrelevant when the issue is purely anti-personnel lethality. Pistols are largely irrelevant as man-stoppers anyway, all claims as to the “knock-down power” of the .45ACP notwithstanding. The only caliber selection issue that may arise in the future is battlefield recovery, which means choosing a caliber common in your area, or within the U.S. military. The argument can be made however, if you are picking up ammunition off dead bad guys, you can just pick up their weapon too.
For the guerrilla fighter, the possession of a pistol is even more important. Besides the obvious ability to continue the fight despite malfunctions of the primary arm, in the absence of supporting fires, there exists the very real need for the guerrilla to be able to go armed clandestinely, when the rifle would create too much of a visual signal, and result in compromise to security forces.
While I personally choose to carry a Glock (Model 19, since I know you’re dying to ask), due to the time-proven reliability of the weapon, I don’t think it’s some sort of magical Austrian talisman. I’ve carried the M9, the 1911A1, Sig-SAUERs, and even a Makarov on more than one occasion. Whether you stick to the ridiculous old notion of “never carry a pistol that doesn’t start with at least the number four,” choose a 9mm Parabellum, or some off-the-wall cartridge like 7.62×25 or .454 Casull, the subject of caliber and model are largely irrelevant. At handgun distances, they’ll all make a hole. The only issue of importance is developing the ability, through training, of using the weapon effectively and efficiently, under combat conditions.
If you’re still hung up over the issues of caliber and model selection of your primary arms, you’re spending too much time on inconsequential non-issues. Quit worrying about which weapon to use and master as many as you can, in the context of the likely combat paradigm you will be required to use them.