I mainly carry a 9mm for two reasons.
1) it is the common family caliber. We have five XD9SCs in my immediate family. My wife is the deciding factor. She shoots the 9mm best. It is a moot arguement for the rest of us.
2) capacity. 13+1 with two 16 round reloads. 46 total rounds at hand but more importantly 14 initial rounds. This becomes a major factor if you are engaging multiple threats. I want to stop, or at least dissuade, as many threats as possible before having to reload. By the time you have to reload you are probably behind on the curve.
3) concealability is a distant third but still a factor to give serious consideration.
Let's say I am a proficient 2 COM 1 head shooter. That seems to be the "standard" shot arrangement on forums. With my compact .45 I have 7+1. That's two attackers and a couple of rounds left over (let's just say I was off that day and had two misses). What if there is a third attacker? Now I'm reloading somewhere in the mix. Choose your order of shots but realistically you'd better be putting at least two on target and figuring in a miss or two each target.
I realize that the chance of having 3+ attackers is probably rare and if you do you probably are somewhere you shouldn't be anyway. But sometimes, honestly a lot of the time, I am in a location that may bring multiple attackers. If it is just me and my wife out then I don't much worry about capacity and will just as likely carry my .45 because I am looking to cause separation. If I can put a couple of rounds in the target and hot-foot it out then I'm good with that.
On the other hand I put myself in a position to have chosen to defend others if need be. That may involve more rounds than my .45 and I can carry. Of course, have a sobering realization that I will probably never get through the second magazine. Either the threat(s) will be down or I will. Who knows, I may get lucky. I'd rather have more rounds than needed than not enough.
I like the .45. There is something about that BIG chunk of lead that gives you a warm fuzzy. But with today's standards in ammo manufacturing one will do just about a good as the other. We can throw all the physics equations we want at it but it doesn't change the facts that handgun calibers are not that good, especially in the sudden self-defense scenario, at reliably stopping an attack. Multiple rounds helps that defiency.
Right now I tend to choose capacity over caliber (with the obligatory disclaimer of it being a sufficient caliber...).
Now all of this is fine and good as I sit at the computer having never shot (or wanting to shoot) a human. So my thoughts on this are exactly worth the experience I have in this matter...absolutely nothing.
