The 5.56 (.223) is a high velocity cartridge which is really accurate out to over 500 yds with the heavier projectiles. These give the 5.56 more wind buffering capability than the 55 gr FMJ.
On the other hand, the 7.62 has charactistics similar to the 30-30 Winchester. At closer ranges (which an assault weapon is designed for) it is a formadable round but for accuracy at range it is a poor choice.
I have both and given the choice, I would choose the 5.56 hands down. Regards, Mikey.