Missing Context
What's not clear is context.
Day-to-day routine as usual? Infrastructure still in place? Repair and maintenance of chosen gun still available? Everybody still has a job, goes to work, raises a family, gripes about the school board?
In that scenario, something suitable for HD & SD, something that can be carried and even concealed, should the need arise. For this, I'll take something in 9mm, like my Taurus PT911, or a Beretta, or a BHP. Heck, if I had more experience with them, I might actually go with a 1911-patterned pistol in .45 ACP.
Change the scenario a bit, remove repair support, mess with the infrastructure, add an element of having to provide certain things for myself, then we change to a different pistol: a 4-inch .357 like (my) S&W 586-7. Exposure to failures should be substantially lower, and the broader range of ammo covers more applications, from defense to provisions.
Continue to degrade the civilization aspect of things, and I would continue to go with the .357 revolver.
Past a certain point, however, where living in town settings has become a bad idea, where hunting has become routine, where it's simply unthinkable that anyone would go anywhere unarmed, I'd have to seriously consider switching to a .357 carbine. It eats the same food as the revolver, but it provides more reach and accuracy. Having a bigger magazine doesn't hurt, either.