Sig 556 does NOT use the same mags as the AR. Parts for the Sig 556 are going to be harder to come buy.
Hardwarz
Actually the 556 does take USGI and PMAG's just fine.
556
Pro: Proven piston system based on the widely used 551 series, SIG quality
Con: Price, lack of built in sights, heavy, limited after market vendors and parts
A lot of folks are purchasing the 551 forends for their 556's, sadly they can only be had from a place in Europe and aren't cheap.
The S&W MP15 seems to have a fairly well established following, but when you start looking at that price range, you might consider LWRC and POF as options if you are looking at the 556 and a piston driven weapon.
Nothing wrong with wanting it set the way you'll want out of the gate, but given that this is your first AR, what I can say is you most likely don't know for certain what you'll really want. Not saying that to be condescending or rude, but take a few minutes and look at some of the same pictures from the same guys in the pic thread. You'll see a lot of evolution, lord knows my first AR went through about 5 iterations to get to where it is now. That gets expensive when you add up all of the changes and value loss on parts.
So my recommendation is to get a solid base configuration and shoot it. Then start thinking about what else you can do to it to make the weapon fit you better.
You might find that the Troy rail included with the MP15 Tac is too heavy or not long enough. You might find that you really aren't a fan of flip up sights and opt to go with a fixed Front sight and Rear sight, or combination there of. You most likely will decide to replace the stock, grip and maybe the trigger guard. You'll most likely end up with a Vertical Front Grip, Light and Optic of one type or another.
Until you're comfortable with shooting and hitting targets with the rifle, I'd say stick with Irons (good out to 300 meters). Go drop about 500 rounds through it, that should be enough to work out any kinks (e.g. break in) and give you a much better idea of how you want to have the rifle configured.
AR's are like Lego's .. and the choices are just as prolific.
