I think I'd pick the M16 over the AK47. It's first of all a newer weapon and the ergonomics is always playing a big role, when I'm to pick a weapon. The M16 had a lot of problems during it's early days. Especially when it was introduced to the rough jungle climate during the Vietnam war. It was equipped with a 30 round magazine, but it could only take 18... otherwise it was likely to jam. It was delivered as an "no-maintaining rifle" which meant that it should't be cleaned... which proved that it was the opposite. The early M16 needed a lot of maintaining, but since later version have been improved (I strongly suppose), I'd say that this rifle have also become a better rifle. Personally I'd still pick a Heckler and Koch rifle... I just have a weakness for german firearms.
[edit] as someone mentioned earlier, the ballistics of the 5,56*45 is much better than the 7,62*39.