Caliber Wars, the endless debate of 9mm vs .45 vs. .40 vs. .44 is something to behold. It's fun listening to this. I owned a gun in .380 for a while and sold it. Not because of the supposed lack of stopping power. Rather, at the time you couldn't find ammunition to shoot from it. It was to be my CCW piece and as such should not be considered a range gun. I didn't need that much, but I was looking to become proficient with it. But I am straying away from my intent of this post.
Another variation on the theme of caliber wars, is to pick one caliber that will work in both a long gun and a pistol of some sort. People debate endlessly and pick some compromise of a caliber. Usually a bigger magnum or a 10mm. Something in .45 long/.410 sounds ideal, but who am I to say? That's two different rounds you'd have to carry. And among one caliber, you have the choice of SJHP, JHP, RN, SWC, plus others I don't remember.
From there, what gun will you pick to use. Barrel length and twist has an effect on accuracy. That snubby .357 and a matching carbine are two totally different animals. While the snubby might eat anything you put down its barrel, the carbine might work best on a diet of JHP.
The more I learn, the more questions I have. Picking one caliber seems to me to be a starting point to find out what works best, slug and powder wise, in whatever you choose as your two guns.