I've found that fluoro has a little harder and slicker surface. That makes it good for nymph fishing -- more abrasion resistant and less water absorption than mono -- but its also easier for knots to slip -- with the telltale pigtail end -- especially in smaller diameters. I still use an improved clinch knot for tying to flies and tippet rings, but I increase the number of turns from 6 turns with mono in 4x or 5x size to 7 turns with 4lb fluoro (Seaguar Invizix).