I would have said Scout Squad also, until I shouldered the Socom a couple weeks ago. It handled not unlike a Mini 14, little more beefy, but roughly same length and a nice heft. No question 18" would be better, but was just reading an article today, on Encore pistol and rifle bbl. lengths, with surprising chrony results. No way around a disorienting muzzle blast. But it will get you out to 500 yd. (with optics, not possible w/ large size, CQB iron setup), with authority. Double the .223's effective range. And if you're talking about a 16" AR w/ standard gas system, you have a carbine that requires stricter maintenance, and limited stopping power with such a short bbl. Now, a .308 has incredible versatility- hunting, SD/HD, battle rifle, etc. (get a .22 too). I guess the Scout mod. is the better overall choice, I just fell in love with the Socom.
SHTF, would be good to have spare/replacement parts, mags, and ammo by the case, and working knowledge to make your own repairs. With the .308, that's gonna hit hard, if you just look at the price of a rifle.