The Perfect RIfle
Pretty much any rifle you choose will be a compromise.
A 5.56x45/.223 shooter will be lighter and permit carrying more ammo, and this is a good thing. At range, however, it doesn't have a stellar record of performance on big game. On the other hand, for self defense this is a very serviceable cartridge.
A .308/7.62x51 shooter will be heavier and so will its ammo, but you can expect pretty good punch at most useful ranges. Self defense becomes a problem, though, because the majority of self defense scenarios are going to be at close quarters. A "battle of the snipers" is way down the probability tree.
I happen to be a fan of the M1 Carbine. It delivers the sort of punch you would expect from a .357 magnum, and does it at 100 yards. It's not a high-performance round, and any big game will probably require a second shot. On the other hand, it's quite acceptable for varmint-sized game, and it certainly will serve in a defensive scenario. It has the capacity and rate of fire needed for close engagement. The rifle itself is exceptionally light, and the ammo is lighter and more compact that either of the two above.
But, as you said in your article, it all depends on what you're trying to survive.
In a conventional end-o'-society situation, there is the initial chaos and people scrambling to stay alive and/or get the upper hand. Okay, so that plausibly involves "social encounters" of the me-and-my-rifle kind. However, if you find yourself in a shootout of the kind requiring high-cap mags and rate of fire, your strategy has already failed, and your actual problem isn't the wrong rifle, it's a failure to have enough people on your side and/or a failure to avoid such engagements in the first place. Even if your situation is some kind of riot in your part of town, you're not going tango hunting; you will want to use just enough persuasion to keep the action away from your place.
Remember, you haven't parachuted into hostile territory with orders to engage "the enemy" in large groups, and neither has anyone else. You might well have a bigger problem with people who figure that since you're the guy with the gun, you've been elected their guide and protector. Once the chaos phase passes, your primary problems (at least the ones that might involve a rifle) will be random predators and hunting game (assuming you're anywhere near game).
And, speaking of game, what are the odds that you will find yourself subsisting on marmots and squirrels and rabbits instead of deer and elk? Squirrels with a .308? Should be interesting.
In a natural disaster, you're less likely to have to "shoot your way out" of anywhere. You might have to defend your family and your stuff, but probably not in any assault-on-the-castle fashion. Once again, you could wind up with an entourage that you didn't count on. And, once again, if you're out in the country, it's more of a random predator/hunting game thing.
That leaves simple misfortune.
You're driving through a snowy pass and something knocks your truck off the road. You're out fishing and when you come back, your truck battery is dead and you have no cell signal. You've gone hunting and gotten a flat tire and discovered you don't have a lug wrench (happened in Idaho last year). You're out hiking with a friend and he gets hurt and can't walk back out. You're out having a good time with family in the Jeep and accidentally drive into deep snow up to the fenders, and you're well and truly stuck, without any cell reception (had family members do that two winters ago). It's not hard to come up with
oh-crap-what-now scenarios that have nothing to do with the breakdown of society or large scale disasters.
Against things like that, your "survival" rifle will be whatever you keep in the truck.
I keep a Marlin 70PSS Papoose (take-down rifle) in the truck, along with a hundred rounds of .22 LR stingers and five hundred rounds of bulk-pack Federal .22 LR hollow points.
No cell signal? Well, besides building a fire with the other stuff I keep in the truck, I can use the rifle to signal for help. I'm not worried about being short of ammo. If I have to handle random predators, I'm good for most critters, excluding bears of course. Bad people? A .22 rifle will seriously ruin your day, and has adequate range to keep heads down at 100 yards. The Papoose will do just fine for small game, and there's likely to be more of that than large game. Oh, and the Papoose don't really weigh nuthin', either.
I suppose I could carry the M1 Carbine in the truck, but I'm thinking I'd need a beater spare if I were going to do that. My current M1 Carbine is not a beater.
If I could find a nice Marlin 1894C (in .357) for cheap (ha -- mine was like $750), then I'd have a viable rifle for small game, varmints, and medium-to-large game, and certainly viable for the little social encounters. And the ammo isn't too heavy. No, you're not hitting stuff out at 300 yards with it, but that's mostly likely not your biggest problem anyway. I'd sure like to have the dollars to make one of those the "truck gun." 'Course, it's not all that light, and you really can't carry 500 rounds of .38/.357 with you.
Tell you what, if I get a nice fat bonus at work, I might consider changing the truck gun over to something heavier and more expensive, otherwise I'm sticking with the Papoose.
It's more appropriate for the things I'm most likely to have happen here.
Now, it should be noted that I don't live in San Francisco, or Houston, or Chicago, or Miami, or even Denver. I live in small-town-North-Idaho. You see, this way I don't have to solve the get-out-of-Dodge problem. I already got out with my Dodge.