At the time 5.56x45mm NATO was adopted, there was research conducted that showed that most engagements took place at shorter ranges. It was also decided that incapacitating an enemy with a smaller cartridge was better than killing them with a larger one, as it produced a burden for the opposing side. Given that, it was decided to adopt the 5.56x45 because you could carry more of it. For the same weight, you could incapacitate more people with 5.56 than you could with 7.62x51mm (or
However that situation has changed in the recent past - combat in Iraq and Afghanistan has typically taken place in open country where the 7.62x39mm round fired by an AK actually does have advantages, but not so much as a 7.62x51 NATO would - hence many units have adopted new rifles for that role - c.f. US Mk17 Mod 0 (SCAR) and UK L129A1 (which is actually an AR-15 variant: http://en.wikipedia.org/wiki/L...)
I should point out that comparing an AK (7.62x39) to a SCAR (7.62x51) isn't comparing apples to apples - the rounds are quite different.
So the correct answer to this question was the one ESR asked - for who and for what? Short range battles, you'd want an M-16 (assuming the questioner meant the most common 5.56mm variant). Longer range, you might want an AK, but the longer it went, the more you'd really want something designed for 7.62 NATO and that might bring you back to an AR-15 variant