I own 2 kits, a M&P 15-22 and a Sig. The main differences is the ability to swap to a larger caliber upper and weight. For me I wanted something that could be more than "just a .22" (as some people call the M&P and Sig). On my CMMG I have it in a .223, 9 mm in addition to the .22. My wife likes shooting the M&P because of the weight. A AR lower is typically made of metal and in a side by side comparison will weigh 1-3 lbs more depending on the rail,optics, etc. For me I like the "heavier" gun. Having a .22 upper is nice for training. I typically shoot 1,000 rounds a week (or more) out of the .22's I have. The $$ savings over the .223 or 9mm allows me to shoot more.