Both were built on totally different ends of the design spectrum to achieve the same result. A reliable, lethal, battle rifle. The M-16 was designed around extremely close tolerances to achieve as much accuracy in a mass produced battle weapon as possible. The AK-47 was designed quite the opposite. It's design was based on quite loose tolerances by comparison to the M-16 to achieve superior reliability. Both models accomplished their goals, albeit the M-16 does it with more accuracy, but at a trade off for reduced reliability. The AK-47 achieves superior reliability, but with much less accuracy.
The bottom line on these guns is that it depends on the type of warfare being fought, and at what distance you expect to engage your targets. In Viet Nam where most of the fighting was close range jungle encounters and ambushes, in that scenario the AK-47 was far superior to the M-16. The inhospitable rainy, wet, jungle conditions also favored the AK-47. Rust, dirt, and mud effected it's operation far less than the M-16 which jammed frequently, and it required field stripping much of the time to clear it, and get it up and running again. Not exactly the best thing to have happen in a fire fight in the middle of the night.
In Iraq, where many enemy encounters are at longer ranges because of the open desert, the M-16 has a definite range advantage, but it's smaller .22 caliber round is handicapped. Much has been reported about the ineffectiveness of the 5.56 MM round at ranges greater than 300 meters. This has been addressed with different ammunition recipes. The jury is still out on it's effectiveness.
So, in many ways the argument continues as to which is best. Both have been in service more than twice as long as the soldiers carrying them have been alive, which alone says something as to their effectiveness as well proven battle weapons. Bill T.