I've always said AR for the same reasons stated above. Despite viscious sentament from the jamming problem the original M16's had, the problem has long since been fixed. I don't hide under mud, so the cleaning problem is not an issue. Under normal field conditions - yes, even blowing sand - the AR/M16 functions fine.
That said, I voted AK.
For some odd reason, my behind-the-seat gun has been my SSR-85b Polish AKM since the first day I shot it. I get 6" groups at 100-150yds, and I've got a sweet 4x24 PSO scope on it.
Despite other shooter's opinions, I find the open sights easier to use when quick reflexes are called for.
I stockpile much more ammo for the AK due to the cost and availability.
Parts, if they were ever needed (see above: reliabilty), are readily available as AR parts, but are much cheaper.
As for SHTF, I'm hidin. If the 'enemy' is farther than 150yds, I'm stayin under my rock and keepin quiet. Thinkin I'll be courageous is a nice thought, but I'm not relying on it.
First rule of survival: If you don't
get in a fight, you won't
die in a fight.