Ya know, I just glossed things over with a quick post about the Ruger LCR because, well, I want one.
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But stroo is absolutely correct--and it sounds as if this is the way you're looking at the issue--that purpose is the most important consideration. It's pretty much inevitable with guns, ammo, or holsters that, when you ask for a general recommendation, folks will respond with, "This is what I have, and it's great." Understandable.
Probably the biggest consideration is whether the pocket gun will be intended as a standalone carry or a BUG. Personally, I'm content with a mouse gun of .32 ACP or larger if it's a BUG. I believe most people think of a BUG exclusively as a weapon you go to if your primary malfunctions. While that's part of the rationale to carrying a BUG, it isn't the greatest consideration.
Nope; the greatest need for a BUG is as a force multiplier when you can't get to your primary, or your presentation or use of your primary has been fouled. I've been on a soapbox about the whole shooting distance and combatives thing for years now; everybody's tired of me beating it to death. The fact remains, though, that if you ever have to use a handgun for self-defense away from you're own property, the range from you to the bad guy will be--with a very high statistical probability--from 7 to 0 feet. It isn't difficult at all for an attacker to foul the draw of a concealed handgun at clinching distance, especially if the gun is carried in a poor tactical position like small-of-back. If you ever get the chance to do any force-on-force training with Airsoft or Simunitions, it's
very eye-opening.
My primary is a 1911 in .45 ACP. I almost never carry it on my strong-side hip without having a mouse gun BUG in the opposite front pocket. If a self-defense situation comes to in-fighting, and it likely will, I don't know which of my arms I'll be able to free first. If I've swept my cover garment aside and have gone for a pistol holstered on my right-hand side, guess where the bad guy (or guys, plural) is going to focus.
Remember the Clint Smith maxim: "A handgun is something you use to fight your way back to the rifle you should never have left behind in the first place." Well, that's how I view a BUG. It has little to do with malfunctions or out-of-ammo situations (that's why I carry spare mags), and it has a lot to do with possibly having to fight my way back to--or fight to retain--my primary carry gun. For that reason, I'm not really concerned with my 25-yard accuracy with a BUG, and I'm a little less concerned about caliber. What I want is something small, that I can access fairly quickly with my off-hand, and that I can pump out several shots at very close range. With a .45 on my right hip, it's almost always a Seecamp .32 in my left front pocket. (I have a P3AT, also, but it's proven to be unreliable and is never carried.)
If a pocket gun is to be the primary carry on any given day, I'm a lot more concerned with caliber and accuracy. I never carry the Seecamp by itself. For that purpose, my personal opinion is that .38 Special or 9mm is the floor. I have two pistols for that use: a Kahr PM9 and a Glock 27 (fannypacker is right: if I didn't have the PM9 and had some extra coin to spend, I'd like to have a Rohrbaugh). The Glock, for me, is purely a cargo-pants or jacket pocket sort of thing; just too thick to carry in a regular pant pocket.
For some modes of dress, sometimes even the PM9 is too large to be easily carried in a pocket. Rather than resort to the Seecamp and go below 9mm, I'll carry the Kahr in a SmartCarry or a 5.11 holster shirt.
In the scenarios I've mentioned, one failing of an auto-loading pistol is that its action is susceptible to fouling if fired surrounded by clothing or other items. Too, if you end up in a grappling situation, you don't want to shoot an auto-loader when its muzzle is in direct contact with the target: it can fail to return to battery. So the real possibility exists of the semiauto in these conditions becoming a one-shot option.
That's where a little revolver--with a shrouded hammer--would come in. The new Ruger LCR is about the same size as a Smith J-frame...a little over 6-inches long, a little over an inch wide, and about 14 ounces unloaded. Inside a special-purpose carry purse or for cool-weather carry in a jacket pocket, if you're being "interviewed" by a potential bad guy and think something might come down, you can have your hand on the revolver--without giving away that you have a gun or taking an aggressive stance--and be ready to shoot through without taking time to draw.
A handy reference for the sizes and basic specs of small handguns can be found here:
http://www.mouseguns.com/PocketAutoComparison.pdf.
Have fun choosing!