There's nothing wrong with either weapon, but both have their limitations.
The AK (7.62 variant) probably won't win too many long range marksmanship contests, not that most of the conscripted users will ever notice or attempt to engage in.
Stock AK's have the weight of a boat oar but the boat oar has better ergonomics than the AK for what it was designed to do.
The AK's trigger, iron sights, optics mounts, and buttstock configuration clearly weren't designed with accurate rapid fire in mind.
The AK can run without lube or cleaning.
The AK isn't too picky about the ammunition you feed it. If you can chamber it, it'll fire it.
The AK magazines are pretty robust... and heavy.
The AR probably won't survive the type of abuse that the AK would, but it was designed for soldiers and civilians who don't abuse their rifles.
The AR has a few small but important parts that the weapon won't function without. It makes carrying spares pretty easy, but you have to have the spare parts.
The AR requires lubrication. It can run dirty, but it can't run dry. That shouldn't be too hard to understand, but apparently it is.
The AR functions best with high quality ammunition and magazines.
The ammunition and magazines that the AR uses are lighter in weight, permitting one man to carry more ammunition for an equivalent weight. Lots of gunfights in Viet Nam abruptly ended after the NVA or VC ran out of ammo. The Russians took note of this and adopted the 5.45 AK's shortly thereafter.
If you have an Aimpoint Comp M3 or M4 mounted to either one, a quality adjustable sling, and working magazines I'm not sure there's a distinct advantage to either platform, apart from the obvious weight issue. The AK-74 or various 556 chambered AK's are more comparable to the AR's in the weight department.
AK's, AK magazines, and AK ammunition weigh more than AR's, AR magazines, and AR ammunition. Once durable plastic lowers are perfected, the scale will tip even further in the AR's favor.
If you are a modestly accomplished marksman and know how to maintain a rifle, I'm not sure what the AK offers over the AR apart from lower cost of ammunition.
The price point of a Colt AR6720 and an Arsenal SLR-101S are about equal. I own both and both cost a little over 1K at the time of purchase.
Bulgarian polymer "waffle" magazines cost more than MagPul PMAG's and are not as plentiful. I prefer the new technology polymer magazines over aluminum and steel, both for the weight savings (in the case of steel) and the reliability/durability advantage (in the case of aluminum).
If the US military demanded AK's, I'm sure that they would have them. There's been no such request made by rank-and-file combat soldiers that I am aware of. Our guys sure seem to kill a lot of people carrying AK's though. I guess all the dead commies and terrorists didn't get the memo about how unreliable and underpowered our AR's are.
The AK ammunition and magazines were, as in "in the past", substantially less expensive than AR ammunition and magazines. That was a major selling point. Now that prices are comparable because of various importation laws and people going bonkers over potential legislation the prices don't seem to favor the AK's by the same margin that they did in the past. Hopefully prices for AK's and AK ammunition will return to sane levels in the near future.
In the end, if ultimate reliability is most important to you then get an AK. If availability of spare parts, magazines, ammunition, accessories, ergonomics, modularity, and relatively simple gunsmithing are important to you, then get an AR.
If you manage to survive an encounter with someone armed with a similar rifle, could you take his weapon if you decided you didn't like your own after the encounter?