I've attempted to cover all bases by collecting as many calibers and platforms as possible! i.e., it's my excuse to buy cool toys that I don't technically need, and in fact seldom even fire, but enjoy gazing upon in my storage area.
As a case in point, I just bought a new underfolder AK built on a Hungarian parts kit. My excuse for this one: "Well, I don't have an underfolder in my collection, I don't have a Hungarian AK in my collection, I don't have an AK with the blonde wood furniture in my collection" and that's all it takes to make my wallet several hundred debtbucks lighter.
But, in all seriousness, I kind of liked the idea of the underfolder as a compact vehicle gun in case I suddenly found myself in a Reginald Denny situation (or Ferguson...or Baltimore...or <insert name of next incident>). I got one of these:
Didn't mean to threadjack, so back on topic:
My thoughts on ammo choice are that it probably doesn't matter nearly as much as we like to think. Be competent with whatever you have, and have at least 1,000 rounds for it. In a TEOTWAWKI situation, you'll most likely end up foraging for new weapons and ammo along with other basic necessities. I think it's very sensible to stick with calibers issued by major military powers, because there's a whole lot more of it out there.