Don't worry too much about caliber. Carry the gun you feel confident with and can shoot well. In civilian self defense, its EXTREMELY rare that a smaller caliber will fail you where a larger caliber would have worked. The rarity of being involved in self defense at all combined with the rarity of being in a situation where caliber would actually matter makes the chance of being under powered in self defense almost impossible.
The only thing I recommend against is .22LR because both guns and ammo tend to be less reliable.
I originally carried a G19 (9mm) but now carry a G42 (.380) because its smaller and more comfortable to carry. This was after doing hours of statistical research and learning that "stopping power" in terms of civillian self defense is a combination of marketing and internet commandos repeating misinformation. In reality, it just isn't that important.
Anyone can tell you stories about the guy who fought though 10 shots with a .22 and use that one incident to conclude the .22 is weak and useless in SD. I can tell you a story about a guy who fought through 10 shots of .45. We hear about those stories because they are rare and interesting, but in reality there are up to 3 million justified self defense uses of firearms in the US every year (CDC, 2013), most actually involve a .22LR (FBI uniform crime report, 1981-present), and only a handful of those 3 million incidents involve an attacker fighting through being shot.
Its different for police and military and civilians rely too much on them when talking about self defense. For them, stopping power IS important because they are likely to be shooting across longer ranger, through barriers, against body armor, and may need to drop someone quickly. For civilians, 99.9% of the time you are shooting a soft target a few feet away from you. A bigger gun may give you a slight edge in a some situations, but you also need to consider flinching, overpenetration, hearing damage, and mortality associated with larger rounds. I say mortality because killing someone is a serious matter both legally and psychologically. If a smaller round is just as likely to stop an attacker while being far less lethal (it really is), it makes sense to carry the smaller round.
In conclusion, carry a gun that is reliable and that you are comfortable with. I highly recommend the Glock 42. The ideal carry gun in my opinion is a double action revolver with some sort of "snag-free" design for the hammer. If you need a larger round to feel safe, that's fine, but understand the consequences (overpenetration, issues related to killing someone, increased size and weight, harder to shoot accurately) and most importantly, understand that all available evidence suggests that size really does not matter when it comes to civilian self defense.