I have a well-made AR-15 and a Saiga .308.
I enjoy both. I want an M1A someday, but I like being able to buy 100 rounds for $40 for reloadable .223 ammo. The .308 is around $43 for 100 rounds of non-reloadable ammo [or about $67 for 100 rounds of reloadable ammo].
My ammo prices are not based on buying bulk ammo [250+ rounds]. My .223 price is WalMart 100 round Federal box pricing. My .308 is from Sportsmansguide by taking 20 round boxes times 5.
I don't reload yet, so getting non-reloadable saves me money and I do it. I also save brass for when I do reload.
I will say this: .308 feels VERY satisfying to shoot, but I can shoot more .223 ammo for longer with less pain to my shoulder and my wallet, and more shots on target.
Someday I'd like M1A, as it would be better for MOA shots, but the Saiga .308 will work fine as a SHTF .308 rifle.
In your situation, I'd probably go with the .308 and .223 rifle combo over the .308 only route. But, I wouldn't go .223 only ever.
Oh, I also have other bolt/lever action rifles for the precision w/ a large caliber, that is why I went Saiga [ak action] .308 instead of Ruger Gunsite Scout rifle: ability to throw lead down range in a hurry.