My personal preference is toward the .308 round, because of the power of the 30 caliber cartridges and availablity of the ammunition and the variety. I also tend to lean towards firearms that I will have in my home if TSHTF. In other words, ammo availability for the 6.8 will be almost nill, unless you are set up to reload in large quantities or have purchased several cases at that time.. But there will be plenty of 5.56 and 7.62x39 and 308 in the streets or in the market. Same thing with Handguns. That's precisely the reason why I do not put a great deal of emphasis on non-popular calibers for handguns that I intend to keep, such as 45GAP, or 10mm. Good cartridges, but limited supply. Brief example is a friend who was away from home during 9/11 like I was. He had a 10 mm pistol and one magazine. He was concerned about what might be happening and tried to purchase 10mm ammo and a couple of extra mags. It was a no go. He couldn't find them anywhere. He did however say there was plenty of 45 ACP, 9mm, 40 S&W, 38 sp. 357 mag, etc.
I also do not put too much stock in people telling me the 5.56/.223 is not a good round for anti-personnel duty. The primary reason for recent reports of the 5.56's poor stopping performance, was because soldiers in the sand box were being issued M4 carbines with 14" barrels and the 68, 72 and 75 grain bullet rounds. The .223 gets it's stopping power from the shear velocity and damage caused by the lighter weight bullets. In fact, as soon as many soldiers were issued the older 55grain bullets, the complaints stopped. Also many of the Marines were still shooting the 20" barrel infantry rifles and had no complaints. When you cut barrel length, you cut performance, then you add heavier bullets and you start having problems with low velocity. I've seen the damage caused by 55 grainers and have no doubt as to what they are capable of doing. As a close to moderate range rifle within about 200 yards, it performs very well.
If I was strictly punching paper or hunting and was willing to walk away from the gun when troubled times come, then that's different. But the guns I plan to have and hold for good, will be the more popular calibers.That's another reason why I think everyone should own a 30-30 and a 22 rifle. You can find it in any Mom and Pop store, hardware store, and some gas stations.