I have the Orvis Guide sling pack and the Patagonia Atom Stealth sling pack. I use the Orvis pack for tons of stuff outside of fishing - around town, traveling, biking, skiing, hiking - and I have gotten several inquiries about it. The Atom Stealth is small and compact, and a nice pack. I will often say I want to go minimalist, but by the time I throw in my fly boxes, tippet and leaders, a few gadgets, a water bottle, some lunch, a small first aid kit, phone, maybe a small camera, toilet paper, wallet and license, and some sunscreen or what not, the packs fill up pretty quickly. I like both, but I would give the nod to the Orvis due to its larger capacity. I'vd never found myself saying "gee, I wish it didn't have so much room"; I just don't fill it unless necessary. Probably about 50% if the time I use most of the volume. Both packs are extremely well made, are comfortable, and are backed by companies with great customer service. I'm 5-11 200 and both packs fit fine.
Good luck!