I'd say the Colt clones and Ruger New Vaquero with 5 1/2" barrels are most prevalent. S&W Schofield's are getting popular as are Colt and Remington C&B's. Most seem to want their rigs period correct as in no drop loop or fast draw holsters. Also as you probably know good cowboy holsters are thick, stiff leather, 8-10 oz. Mexican Single or double loop like this Cheyenne single loop are popular only in a double rig.