Glad to see its not another basher thread ,i think they are both great rifles, but each with its own flaws. I own a newer mini 580 series and love the rifle, its never failed in anyway and while its not as accurate as an AR its as accurate as i need. I dont use mine for HD but i guess it could fit that bill ,as far as mags are concerned i just bought two 30 rounders(factory) at a local show for $29.99 each, while not cheap ive seen them close to double that.
I don't own an AR but have fired a couple and researched them , coming close to a purchase (bought a Rem 700 .270 instead ) They are accurate and reliable 98% of the time but definitely are the GO-TO rifles of the day. As another said they are like Lego's and can be built to your suit, ive seen some Bad ass looking AR's on this forum.
I don't think you can go wrong with either rifle, have fun shopping and don't forget to post some pic's when you get your new toy.