Like many folks, I started out fly fishing with just a rod, a reel, and a few flies. Someone eventually gave me a vest. I enjoyed the vest, and used it for years. I could carry just about anything, and hooking up a net and carrying a gatorade was no problem. Like many things in life, I thought there might be a better answer. I tried out a chest pack. I used it once and that was it. If you can benchpress a decent amount of weight, the chest pack is not the answer. At least it wasn't the answer for me. Then I switched to a lumbar waist pack by Simms. This pack was pretty cool. I used it for a few years. I could carry all my fishing gear, plenty of liquids, etc. After a while, I started using my lumbar pack as a sling. Of course, the lumbar pack wasn't designed as a sling.
Earlier this year I was presented with a birthday present - an Orvis Safe Passage Sling. The lumbar pack is now retired. The Sling pack is perfect for me. It carries everything I need - multiple fly boxes, water, tools, tippet, pliers, etc. The pack doesn't get in my way while I'm fishing - it fits as snug as I want it. I can move it around no problem. I'm sure there are other great slings out on the market - I'm quite happy with the Orvis. I'm not entirely convinced that a sling is better than a vest, but I for me, it's better than the other options.