Niether -
I see no good reason to buy a Sig 556. I don’t see that it offers anything that a well built AR doesn’t already give you? If it’s the piston driven aspect you like, I would still wait because I believe there are better options either currently, or soon to be, available.
I also see no reason to have as many AR's as you mention. If you have the money to burn, great, have at it and build\buy as many as you wish but judging by your post here, it would appear that you don't have that kind of money. (Don’t sweat it, most of us don’t) When budget constraints are an issue, it doesn't make sense to me to have a number of weapons that basically do the same thing. As an example, what would be the reason\thinking behind having both a 14.5" and 16" M4gery?
I would try to limit your AR collection to no more then 3 or possibly 4 total. Really, I think most people could get away with 2 for probably about 99% of anything you would want to use an AR for. One would be a business oriented carbine for close in work (>100yds) and the other a rifle length AR setup for longer range work (100-300yds). Anything above those two is just gravy IMO.
The best use of the rest of your money would be to use it for training, perferable a formal class but at least for ammo\mags\range time to practice with. If you are looking for another firearm, how about something different to round out your tool box? Maybe a larger caliber battle rifle (M14\M1A, FAL, AR10, etc) or maybe a long range precision rig or maybe a tactical\combat style shotgun?
That’s just my opinion, only ‘cause you asked