Over at Unc's place, there's an amusing little poster whose aim is to further the caliber wars. Just to catch anyone up who wasn't munching on popcorn whilst perusing gun nut forums for the last decade-ish, there's a disagreement as to whether it's best to have a lot of rounds of a smallish caliber, or a few rounds of a biggish caliber, usually simplified as 9mm vs .45 acp (or .40 S&W). PapaDeltaBravo has a pic to illustrate the smaller caliber advocates position.
A 9mm loaded with modern JHP's of a sufficient velocity (which you'll find in most "defense" loading of said cartridge) is probably fine for most situation where someone would have occasion to fling rocks at someone. Most people will be well served by that cartridge.
I don't carry a 9mm. I carry a 10mm. (cause it's 1mm better!)
Looking at the pic PDB has up, you'll see that penetration of several different projectiles is virtually the same, thus leading one to conclude that since each projectile is practically the same as another, then using a projectile in a cartridge with increased magazine capacity and reduced recoil would be a wise decision.
That pic however is not all inclusive. I note that the .40 S&W rounds weren't loaded to their full potential for example. And it was not all inclusive, as the 10mm among others wasn't shown. Also, I presume it was bare gelatin.
In WW2 (and The Korean War) the most prevalent round on the battlefield was not M2 ball. It was M2 AP. That's because there was a helluva lot of metal all over the place, and having a projectile that could reach the other side of some of that metal was desirable. M2 ball would have worked just fine on the itinerant fascist, imperialist or (later on) communist soldier who hadn't seen the wisdom in surrendering, but it had to reach said enemy combatant before it could do its job, and with the metal prevalence it was best to just go ahead and use AP ammo.
That's what I think a lot of folks neglect when proclaiming their choice in cartridge, especially when accompanied by "stopping power is a myth" or other such rallying cries. I'm sure 9mm will do fine if its trying to reach vitals covered only by denim and skin, but what if there's a more substantial barrier?
Bears live out here. There's been occasion when they've wandered into my neighborhood, and I'm only 15 minutes from downtown Denver. A 9mm won't reach anything terribly important in a bruin. It's arguable that a hot loaded FMJ in .40 S&W wouldn't do enough to put a bear down. A 10mm loaded relatively hot will.
I know my luck well enough to speculate that if I am attacked in such a manner that I think making real loud noises whilst throwing rocks is warranted, that the assailant(s) will likely have more than just a winter coat on. Depending on how thick any body armor is, a 10mm may not penetrate though it'd be more likely to than a 9mm (though less likely than a hot .454 Casull). But the loads I carry have more muzzle energy than a 9mm, thus a better chance of distracting an attacker even if the hit doesn't go through body armor. I may even be fortunate enough to crack a rib, which will make that assailant less effective in attacking me.
For any number crunchers out there, I found this test of several different types of 10mm ammo. I'll note that all but two type of 10mm achieved at least 12 inches of penetration (the two that didn't had more than 100% expansion). Here's a ballistic chart showing velocity and energy of various 10mm loads. (And for what it's worth here's a forum thread discussing 10mm ballistic gelatin tests).
You can find 10mm pistols with capacities varying from 2 to 20 rounds, with concealability decreasing proportional to the yield of the cartridge box. In general it's usually about the same number of cartridges in an otherwise identical pistol chambered for .40 S&W, and two or three less rounds than you'd find in a comparable 9mm. My 10mm's have noticeably more recoil than a comparable 9mm, and thus follow up shots are a tad slower. Accuracy is equivalent for all practical purposes.
Where the 10mm shines is that it can go places the 9mm or arguably even the .40 S&W and .45 acp just can't go, namely to the vital zone of something with a moderate to heavy barrier twixt you and it. (A .44 Magnum would accomplish that even better, but at the cost of increased firearm weight, increased recoil and much lower capacity.)
I mention all this to illustrate that when most folks start pointing to that PDB pic of gelatin tests or otherwise proclaim the 9mm equal or superior, they're neglecting the qualifiers, such as having the target being a human sans any sort of artificial barrier.
If you carry a 9mm and are happy with it then cool. I won't attempt to alter your selection. If you carry a .380 acp, a .22 magnum, a .357 Sig, a .455 Webley, a .32 H&R Magnum, or even a .22LR then as long as you're proficient with it and understand the cartridge's capabilities and limitations then that's groovy.
There is no one best cartridge, only cartridges better suited to particular tasks than others. a large part of what determines that suitability is little more than personal preference. If you lack bruins and discount the small but not improbable likelihood of being attacked by armor wearing thugs (here's where I'll remind y'all of the Tyler Texas courthouse shootings, and Mr. Wilson), then the 10mm may not be the best choice for you.
For me and the way I think, the 10mm makes the most sense in a carry gun. However it's still a compromise and if a bruin or armored miscreant was something I knew I was gonna face, I'd grab a Garand loaded with AP. (that's if I couldn't get my hands on a Bofors of course).