The Ruger AR 5.56 seems to have a tighter "fit and finish" than either the S&W Sport or Sport II. IMHO, this means that as the gun "wears in" that there will be fewer places for dust and other debris to collect.

If it isn't too expensive, Ruger's trigger upgrade may be worth the investment. The whole idea is that I won't be buying more than one AR and I want it to shoot well until I stop breathing. After that, I won't care too much, will I?