I used a vest for years and got a sling after fishing with a fellow who used one. He liked it and it looked like it would work. I bought a sling bag (not Fly Fishing specific) and tried it 5 times. Twice on the Creek and three times on still water. I am back to my vest. It always seemed to move around at the most inconvenient time. Even when it did not bump into my casting arm, just the sudden movement was enough to take my attention away from my casting. I even tried on both shoulders with no real difference. I also tried a "fanny pack" that didn't last as long as the sling.