AK: Simple, robust design. I'm not a fan of the sighting system, nor of the AKM stock; when I shoot, I prefer to get a good cheek weld so that I can look through the sights. The rifle's ergonomics can be better, but can be mastered with practice. Some AK models are more accurate than others, particularly if said models are not of the 7.62x39mm variety. They are reliable, but remember that all manmade things can break; take care of your AK and it will take care of you.
AR: Simple, robust design, WHEN DONE CORRECTLY. Ergonomic, modular, accurate, reliable, easy to use, even with irons (Irons are all I run, for now). Most of the time, you just add lubrication and you're good to go. Excessive cleaning is not necessary; a good wipe down and a couple of passes through the bore with a bore brush is all that is really necessary for most cleanings.
My viewpoint; bear in mind that I have far more experience with AR-style rifles than AK-style; I have enough time on the AK to have an opinion but not enough to know the nuances of the rifle in and out.