I have a bit of an obsession with these packs and have had the Patagonia Stromfront backpack with the waterproof zipper, then the same but in the sling version, and now I have the Stromfront rolltop backpack.
I usually always carry my Canon 7D so that is why I really enjoy a watertight bag. What I didn't like with the original Stromfront pack is that it didn't have a waist belt on it, the new version does but it also retails at $365 in Canada.
So I then gravitated towards the sling as it looks really cool and is not as much $ as the backpack. I also thought it would be better for casting. That is not the case however and you are better to lay the pack on the bank if you're fishing a big run...
The sling works great as long as you are not putting a lot of weight in it. In my case with a DSLR camera and a full nalgene bottle, plus fly boxes, it was quite heavy. Seeing as all the weight is on one shoulder, you feel quite unbalanced. It makes more sense to go for the backpack.
So now I use the Stromfront rolltop. It's quite a bit less money than the zipper version. Waterproof zippers are very expensive. Secondly these waterproof zippers aren't that easy to open anyways, so you are not giving away much convenience by opting for the rolltop.