I would go with the AR15. I would definitely go with one that uses gas piston, instead of gas impingment. Gas piston runs cooler, is a lot more reliable, and cleaner. I was never a AR fan until I ran across LWRC. I was recommended to check them out, and l found a guy at my local range who owns one. I fired at the range, and fell in love with the way it works. I bought one for myself and took it out to a friends ranch and put it through the same torture test I ran my ak through it came out performing just as reliable as my ak. It does have the advantage of accuracy, and distance.
I also believe that ammo is going to be more available during a zombie apocolypse. On top of that one thing a lot of people dont think about is what happens when your weapons system fails. Parts for the AR will definitely be easier to come by on the road than parts for ak's.