I'd go with the AK. The M16 is more accurate and has better sights, but the M16 fires a lowly .22 caliber round that can't knock down hardly anything. If the enemy is behind an oak tree, the M16 is worthless. The AK's 7.62 mm round can punch through the oak tree pretty easily while the M16 cannot.
The M16 is plastic, and thus if you whack an enemy with it it will break. The wooden AK stock will break the enemies skull instead of the skull breaking it. In close range, the AK far outdoes the M16. IN a long range open field firefight, the M16 will have the edge.
But I would rather have the M1 Garand than either. Very accurate and has a much larger bullet than both (.30-06, though the bullet itself is as big as the Ak (7.62) the casing longer (63mm vs 39mm) and has more powder which means better range and accuracy). Semi auto means less wasted ammo. Aimed shots >>>>> Spray and Pray.
If I had to choose a modern rifle, I'd go with the FN FAL (it uses a 7.62x51mm round, just between the AK and M1).