AR's = Trendy and popular at shooting ranges because movies, and ANYTHING that was used by our military is instantly SUPER popular with everybody that just wants to play soldier.
AK's = Most reliable weapon you'll ever own, enough of an improvement that when our troops in Iraq had the oportunity (mostly special forces teams, contractors, or the like were the only ones allowed) they used AK's and left AR's at the base due to better penetration, and the mini-AK's they were finding availlable being more practical from in a vehicle, more reliable, and better for clearing rooms. All around a better weapon unless trendiness, or shooting rodents is the priority.
Remington sells brass reloadable 7.62 x 39 ammo I've seen most everywhere in a mostly white box with green labels, its the same price as the .223 ammo they sell for a box of 20. I think they call it UMC or something like that. They list it on their site though.
AK's and AR's really cant possibly be compared to one another. Ones all about form, ones all about function. Just depends whats more important to you. IT"S YOUR MONEY BEING SPENT.