They both have some pros and cons, but the AK comes out on top.
The main advantage of the AR platform is that it is very accurate because it is built with strict tolerances, this also is part of its downfall as sand and debris can easily collect in the reciever and cause the weapon to jam. It also uses direct gas impingement which vents gas directly onto the bolt carrier.
The AK's gas system vents gas onto a piston that drives the bolt carrier backward, preventing the gas, soot, and fouling to end up in the bolt assembly. Also, the AK is built from stamped, cheap parts with very loose tolerances. While this does make it less accurate than an M16 or M4, it also allows for debris to get into the rifle without affecting the mechanisms as much.
Also, the fact that AK's are inaccurate is partially myth. While they are not as accurate as the M16, they are by no means completely inaccurate. This myth comes from the fact that the AK has its rear sight mounted on the forward handguard, this type of sight arrangement isn't as familiar to most western shooters, hence the common sentiment that the AK is not accurate.