Kinetic energy is what is important, commonly called "knock down" power, even though common physics prove bullets don't knock people down. If a bullet stops in the body, it is far more likely to kill you due to the kinetic energy, but if a bullet passes through you, it doesn't hold this same energy. For instance, many people compare 9mm to .45 ACP. A 9mm is a fast moving high velocity round that is quite small. When it hits a person, it doesn't always stop in the body. This is why most people use 9mm hollowpoints. It ensures they stay in the body. The .45 on the other hand is a big, slow moving bullet. When it hits, its kinetic energy is maintained and more damage is done.
I've heard that while .223s are really fast (meaning it would pass through the body) and small, they tend to tumble and shatter in the human body, which does quite a bit of damage. Since it is about 10 rounds more in each magazine for a .223 over a .308, I think I'd stick to .223. But as a Navy Seal once said, "When you give them two in the head and one in the chest, they won't know the difference."
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