Optic Selection for Sig 556
It is hard to beat the Bushnell Elite 4200 1.25x-4x scope for an all-around rifle optic. It is built like a tank, features an illuminated reticle that uses a common battery, is waterproof, has an excellent all-weather lens coating, a fantastic warranty, and is a good value for the money. It really is a solid variable optic. Is it as tough as an ACOG or Aimpoint? Probably not, but it also costs significantly less and offers more flexibility than either. I also don't plan on heaving my rifle over a mountain anytime soon. It will easily withstand field use.
If you can't afford a LaRue quick-release SPR mount for it, check out the Burris PEPR...it works just as well but is screwed onto the rail. The 1.25x mag is fine in close quarters, and the 4x definitely helps with precision. Unless you plan on kicking doors in for a living, or rucking through Afghanistan for extended periods of time, a variable optic is the best way to take advantage of the 5.56's capabilities.
BTW - the rotary diopter iron sight from Sig is a superior sighting system. It is solidly built, easy to use, and very precise. Definitely worth the money. I wish my AR came with one.