It depends on the situation, the M-16 has a better range in both automatic and semi automatic modes, but the AK-47 requires less maintenance and is designed to fire reliably in muddy, wet and frozen conditions of the Soviet front line, so in an actual "wasteland" situation the AK may be better suited.
For the game though I think the M-16 looks cooler...

The AK is select-fire. It is capable of a single shot per trigger pull. The M-16 has good low temperature performance. The main difference for reliability is that the AK has larger tolerances, only two locking lugs on its bolt, and the bolt rides on rails. This means that a bit of grit that gets in the gun is less likely to hamper operation, which is good because the AK action is more open to the surrounding environment. Additionally, the AK's long-stroke action keeps more of the heat away from the bolt which gives the extractor spring a longer life before needing replacement.
Of course the AK comes with some downsides. Looser tolerances means less consistency in how parts mate causing lower accuracy. A long-stroke gas piston adds weight, increases reciprocating mass, adds parts to break like the op-rod, and moves the heat forward to the handguards. The last is minor unless you are shooting magazine after magazine in full auto. If you are, don't be surprised when your handguards light on fire. AK's tend to have pretty light barrels. This youtube link http://www.youtube.com/watch?v=xeeeFxA_9nA shows how it will flex while firing.
I'd probably take an AR over an AK in a fallout-like post apocalyptic world because it is easier to change barrels and other parts on the AR. The AR is also mechanically simpler and lighter. It is less tolerant of dust and fine sand, but this can be mitigated in the most part by my high repair skill.