posed here... "Would you prefer a selection of guns in common calibers that shared ammo and parts? Or would you want a wide variety of weapons in as many calibers as possible so that you could take advantage of anything that might come your way?"
My original plan was to be able to use one caliber in a pistol / carbine combination and ultimately gravitated toward Beretta and 9 x 19mm. As of today, I own two Beretta pistols (both of them in 9 x 19mm) and a couple carbines (notably in .30 Carbine and 5.56 NATO, not 9 x 19mm).
As the collection has matured, I seem to have ended up with something that shoots just about anything. Having said that, it appears, from a recent purchase, that anything in .40 S&W will grab my attention. And, truthfully, I have migrated to the .40 S&W pretty much completely as my daily carry.
The aforementioned purchase? A heretofore unknown to me... a Sig Mauser M2... that I probably wouldn't have noticed except for the Mauser logo on the slide and much less bought except that it was in my current 'pet' caliber. All of that blather pretty much boils down to, I picked a fairly common caliber and now have a pile of different arms that will chamber it. So much for the original plan...
So, in a roundabout way, I started out with a desire to own a couple of guns in a common caliber and ended up with a selection that will shoot just about anything that shows up. Even though that be true, I still believe having two of something is better than having one of something... most times. ;)