I'd be happy to do that Popapi. The Sig556 is a proprietary design from Sig that uses the same caliber as the AR-15 (.222/5.56) and the same magazine (because they are so popular and ubiquitous.) Apart from that the SIG556 is more of an Ak47 knock off or the earlier SIG55X design.
The AR-15's attractions as a rifle have to do with light weight, "mil-spec" parts and add-ons and the fact you can make your AR be YOUR AR by the simple expedient of changing out parts and mixign and matching them. This is something you cannot do with a SIG556. This does not detract from the SIG556 design as to its reliability, accuracy and ergonomics; it's just not an AR-15.
If you want a taste for what you can do with an AR-15, get the Brownells catalog dedicated to ARs, it's in its fourth edition now. You compare it to the same catalog for a SIG556, this one would be much smaller and would only contain the parts that you can attach to the rails, such as scopes, lasers, etc.
One last item that really turned me off the SIG556 is that they make the upper be the numbered part; the one that has to go through an FFL. For the AR-15, only the lower is the numbered part; With one lower, you can have as many uppers are you want and you can buy them over the Internet and have them delivered to your door. So with that lower, you could have an SBR, a carbine, a target rifle and a space gun. You could even have a piston-powered upper if you wanted to. You can even have uppers in different calibers.
The AR is far more versatile than the sig556.
