You need an assault rifle, you have the choice between a 7.62x51mm or 5.56x45mm weapon of your choice.
Which caliber would you choose and why?
For me, it's 5.56x45mm for one simple reason, more ammunition per magazine and more ammo for the same weight. The G36K/G36C and XM8 being prime examples of my kind of AR
Although I've always been interested to see what a intermediate caliber like a modern assault rifle could do with a 6.4x48mm or some thing.