Hello. I think perhaps as much as logistics, our personal preferences enter the equation on what we carry with regard to weight vs. size vs. "firepower". A fellow who simply prefers .45 ACP will find plenty of true and meaningful reasons to carry an arm capable of the caliber. Those expecting to face multiple opponents might find good reasons for a higher capacity 9mm or .40 S&W. The gent or lady whose main fear is the "street thug" might opt for something more concealable as might the fellow carrying for protection despite legalities or mandates at his job site. The uninformed might simply believe that " a gun is a gun and a bullet is a bullet" and without concerns for practice, accuracy at speed, or even the handgun's longevity, buy something that's small at the lower end of the pricing envelope.
I can speak only for myself in terms of absolutes.
Unless there is simply no other way, I will not carry less than .38 +P in the typical snub J-frame. This lightweight 5-shot revolver is probably what I'd have if the balloon went up unexpectedly around the house or in my immediate neighborhood. I do carry one speedloader at all times as well, but practice regularly with my carry gun, an S&W Model 642. This is my "always gun" and is often my primary. When carrying a "real gun", the always gun becomes a BUG.
Some folks prefer the .380 ACP to the .38 snub as a carry gun. Most often, the reasons cited are less recoil and greater shots between reloads. These are valid reasons, but I personally cannot bring myself to trust .380 ACP. IF the soon-to-be released DPX in .380 from Corbon performs as well as I hope, I might have to rethink this position. Right now, I'll stick with the snub. That does not mean it's the right choice for everyone.
As this is typed, the snub is pocketed and there's a Ruger SP101 3 1/16" bbl .357 less than two feet from my strong hand. It is loaded with Remington 125-gr. .357 Golden Sabers. This less-than-full-power magnum load was chosen as it's the most potent I can handle accurately and at speed if using but one hand...as could happen in a fight. It is a belt gun due to barrel length and weight.
The old J-frame .38 loaded with my choice of defensive rounds is what I'm armed with most of the time. I suggest that the old gun store "habit" of recommending these to everyone wanting a "little gun for the little lady" is a mistake. These require practice for accuracy, particularly in rapid-fire, and there is some "kick" to them in the lightweight versions. I've shot regularly for 30 years and was a police firearm instructor and even now, I find that I must practice regularly with this choice of weapon to stay capable.
I'd feel well-armed with the Ruger and the S&W and a reload or two as I honestly believe that unless taking on a gang intent on robbery (as can sure happen in some of the larger cities like LA jewelry store owners can attest), we'll either solve our problem in the first few shots or be beyond caring if facing competent aggressors.
That said, I frequently carry either a Browning 9mm Hi Power or 1911 pattern pistol in .45 ACP. Both are usually loaded with Winchester Ranger ammo, 127-gr. +P+ in the 9mm, and 230-gr. std. pressure in the .45, although I sometimes carry Golden Sabers.
This rather plain Mil-Spec is relatively new. I've carried it on ocassion, but not as much as some of my older guns. I normally use 7-shot magazines in my forty-five caliber 1911's. It is not fancy, but has had some minor tweaking. It has not missed a stutter in over 2000 rounds. Whether we choose revolver or autoloader, regular or lightweight, the defense gun must be reliable.
Both are carried in good holsters and using a heavy belt. I've found that the all-steel guns are not tough to tote if using belts/holsters appropriate for the job and have tended toward the steel guns in recent years.
I have no real burning concerns about magazine capacity. I feel plenty confident with 7 + 1 in the .45 and the usual 13 + 1 in the Hi Power. I like the Hi Power quite a lot but the fact that it holds nearly twice as many shells per loading as the traditional 7-shot mag/single-stack .45 was not a primary consideration.
Please do not think I'm condemning or saying that the higher capacity guns are not needed or are useless; I'm certainly not intending to and I do NOT subscribe to limiting "high capacity" magazines only to LE or the government.
The Glock 17 or Springfield XD are about the lightest full-size, high-capacity autoloaders I'm aware of. For folks insisting on these characteristics in a carry gun, these might make very viable choices.
I do have a couple of lighter weight 1911's. These are either .45 Commanders or the Springfield LW 5" 1911. They
are a bit easier to carry, but to me, it's not by much and I do find the heavier all-steel guns a bit easier to shoot accurately at speed.
Neither the Hi Power or the .45 Colt-caliber Mountain Gun shown are either the lightest or heaviest of handguns made. I've carried both of these guns for hours on end w/o problems in good leather and on a proper belt.
Were I told I was going to have to face and fight 5 guys at one time (and I couldn't sneak away or call in the Marines), I'd probably opt for my Hi Power or a Glock 17, but I certainly wouldn't feel "lost" if I had to use an 8-shot 1911. I find the Hi Power and the 1911 5" guns to be the autos I shoot best and still believe that placement is power.
Right now, my "best" answer would be for a person to do some serious soul searching and decide the parameters he or she felt were absolutely essential in their carry gun such as caliber, action type, weight limitations, etc and then be sure that they were willing to put forth the time in training and practice to be competent in the gun's use. When they were, I'd suggest staying in practice and if they liked their (reliable) defense gun, go with it.
Best.