308 because in a true survival scenario you won't have the luxury to buy ammo, basicaly do more with less, kill bigger game for more meat that you can use for bartering.
Also 308 you can carry less shells so i don't think wieght would be a factor; IMO humans adjust really quick a 10lbs 308 rifle would be like nothing after a few weeks of constant carry.
Definitely a Combat Knife or other such knives would be more useful.
After a while we'd prolly ditch our guns and use convential methods like throwing spears, slings and traps.
Assuming its an End of the modern world scenario.