Sig 556, why so special?

I'm in a bit of a delima between the two weapons, the Sig 556 and the AR-15, like many people have been before lol. What makes the Sig THAT much better than the AR to be $300-400 more expensive. Of course the gas piston system is a given, but is there anything else that is really that much of an improvement to the AR? The AR is as said $300-400 with virtually endless possibilities of customization. Another question i have about the Sig is, what's the aftermarket like (stock, trigger, grip...etc.)? I've heard something of the stocks being interchangeable, but nothing of anything else. Thoughts and answers?