The AR15 is the most expensive choice...not fully sold on it yet but hey, I haven't shot one yet. Seems to be more of a precision machine....and I'm not sure if thats what I want.
From what I've seen, dollar for dollar the AR platform is just as good a value as the AK, when you're buying new as opposed to milsurp kit guns and stuff. $600 or so gets you a good, solid weapon. $800 gets you a gun that's easily the equal of any other, where it counts. $1200 gets you an $800 gun and $400 worth of accessories.
The lower end AKs are only cheaper because there are no Romanians chipping AR-15s out of blocks of aluminum with chisels. And you can't get cheap ARs that are built with military surplus M-16 parts. But, at least in my opinion, a $350 AK is worth $350, while a $600 one is worth $600.
Comparing AR-15s to AK-47s... I'd say it's a lot like comparing 1911s to Glocks. Both can be made to run just fine, but you can tune an AR a lot more and mess around with fancy schmancy parts. You can definitely make an AR more accurate, but it'll cost you. And you
can get an AR to run just as reliably as an AK, but it's a bit harder, and you need to choose parts carefully. Whereas with an AK, you just buy it, load it, shoot a few hundred times to make sure it's not a lemon, and you can have faith in it from there on out.
Like I said, 1911 vs. Glock.