Mil-Spec has a thinner outer tube than Commercial. Commercial used to be prevelent in the civilian market but nowadays Mil-Spec beats it out in the market so you can normally find more options for Mil-Spec tubes. Also Mil-Spec tubes are flat on the butt end and commercial stocks usually come with an angled down slant on the butt end. Tapco comes in both versions.
I attached a picture that best shows the difference. It's real slight but the stocks are not interchangeable amongst the two.