I enjoyed using a lumbar pack for several years. It was nice because I could twist it around to my backside when I didn't need to access it, and to the front when I needed something. I tried a number of different ones, and the adjustable straps all seemed to eventually loosen and sag. Not a huge deal, but eventually I wanted to carry more/larger fly boxes for certain types of fishing I was doing. Plus, I would sometimes find myself wading in water over my waist, and would need to carry the lumbar pack in my hand until reaching shallower water.

So, I tried a sling pack. I love it! I can fit a LOT of stuff in it, it is comfortable, is easy to access and move back out of the way, and it sits up higher on my body so I don't have to worry about it getting wet when in deeper water. The pack I use in an Orvis model that has a 2nd strap that keeps the pack from shifting position when walking around.