The M1989A1 is big brother to the original M16A1. It was rechambered to accept the same 7.62mm clip ammo that the AK series of rifles uses so troops could use captured Soviet ammo. Although assault rifle users greatly preferred the American ammo for its damage capabilities, the military implemented this change in anticipation of front-line ammo shortages. With this arrangement, a soldier could kill an enemy and replenish his supply at the same time.
The M1989A1 benefits from a slightly larger magazine size (with no deduction of damage) in comparison to its AK counterpart, and is therefore the better of the two when making use of 7.62mm clips (as made evident by its price tag).