All in all I put the 7.62x39 and 5.56 in the same general bucket. The M4 platform is a better platform than the AK for well-funded, well-organized armies, and the AK is a better platform for illiterate peasants fighting a guerrilla bush war. Both rounds fit the same general combat niche.
If I had to choose exactly one weapon to equip all the US/Russian soldiers with, it'd be a plastic AR-10 chambered in 7.62x51. A lesser-of-evils compromise among all the bad single-platform options. Heavy, but not absurd. Powerful enough, but just barely. Good rifle, so long as you keep it clean. Not particularly cheap, but won't bankrupt the treasury if a couple get lost.