SHTF and for me, I want 5.56. More rounds, lighter carry, easier for all my family (women and kids I mean) to shoot accurately, should something happen to me. Also keep in mind that there is no Geneva in that scenario so any bullets are fair game. This is often overlooked. 5.56 that's not fmj will do plenty of damage, especially if keeping somebody from walking towards you is enough...
You are not going to be fighting against well armed troops. Mostly you are going to be fending off people who want your schtuff and hunting small-ish game. .308 serves no purpose in that kind of scenario where you are not actively looking to take down people from +-500 yards or hunting deer, etc. IMO.
But if it has to be .308, gimme an M1A. Never tried a SCAR17, but those look pretty reliable too.
I generally agree with your first para, But for this thread, the question wasn't really about best SHTF rifle, but if you had to choose a larger caliber. Well, at least larger than 5.56.
Good point on the different ammo types.
I've shot the SCAR 17 quite a bit and I think it's definitely accurate, dependable and rugged. I've seen it beat up so many optics. I was told that it was due to the violent way that the bolt sort of jars the gun forward and down upon going back into battery. I would think that then would apply to most semi-auto .308 rifles. But something about that it was noticed more in the SCAR 17. I think Mrgunsngear also had a video on the SCAR that mentioned the same thing. Dunno the truth of the matter, but in the SSF, I've noticed that the years we had SCAR 17, the optics either worked free or started failing.