Honestly it would depend on the battle which one I'd want!
I picked the AR for all around use though. The .308 is definitely a harder hitting, longer range round, no arguing that. The .223 round however is smaller and lighter so you can carry more ammo (the reason the military switched in the first place). The .223 is cheaper too, way cheaper so target shooting and plinking is not going to break your wallet as quickly. The modern AR's are not that prone to malfunctioning like the first AR 15/M16 was and that bad rap has stayed around since the early days of the Vietnam war.
I would love to have an M14, they're great guns. I'd love to have an HK 91too and I passed on one back in the mid 90's once that I wish I'd gotten. It was a new/unused select fire gun for $2k, I'd have just had to get a stamp. It would be worth a lot more now, wish i had it.