I own a Sig 556 because I'm a nut for piston-gas mechanisms like the AK, FAL, and Sig 55X.
The Sig 556 runs like a dream. It is the smoothest, most accurate out of the box, semi-auto rifle that I have ever owned. The trigger pull is amazing for a carbine.
Of course, there are some draw backs, and the AR wins in this one category. And it's a huge one. Sig has yet to sell any replacement parts to the public. So you can't buy an extra extractor or firing pin if you want to. Of course, ARs have gazillions of parts of all sorts available.
The Sig 556 also has lackluster furniture, but very acceptable aftermarket replacements exist. The Sig furniture is functional, but most people don't like it. The AR isn't too different in this respect, in that replacement furniture is available and one will probably wind up replacing some furniture on their AR as well. So I see this as a wash between the AR and Sig.
The Sig 556 is a very, very sweet shooting gun. It is definitely the best-shooting, slickest semi-auto I've every fired (which includes Garands, AKs, FALs, M1As, Rugers, ARs, HKs). If you do buy one, make sure your budget allows for all the other accessories you will want to buy, which may include a scope, mount, furniture, BUIS, and sling.
