AR in .308 is an excellent option. The biggest and best arguement against the AR (IMO) is that it's under powered. Not so with the .308! Also, this is a very accurate round, and it does not take much to make the AR an accurate weapon.
The AK is super reliable. They are everywhere (ie, all over the world) and replacement parts would never be a problem to find. AK's are accurate enough to do what they were meant to do... hit a human at a few hundred rounds. But the average joe doesn't shoot at people.. they shoot at things. So, sometimes the accuracy can leave some to be desired.
I pick the AR though
On paper, it's a toss up, but when they're in your arms and you're pulling the trigger... the smile on my face is always bigger when I'm shooting the AR.