I own both the S&W and the Sig. The S&W runs like a dream as well as the Sig. The S&W is lighter due to the poly receiver where as the Sig is good ole fashion metal and is heavier. The Sig runs Black dog mags which are interchangeable with some other popular conversion kits. The S&W runs their own, the good part about these mags is that they have a nice spring retractor to load. In most cases the S&W comes with iron sights and the Sig does not come with any optics. My opinion is apples and organges. To Tazisland's point it is almost identical to the 556. You won't be disappointed with either. Good luck!