i agree on durability of kuiu packs.... mine got shredded in short order, squeaked bad after a few months, and for myself, doesn't haul weight as well as the other high end haulers by a large margin. i ended up buying a new belt and suspension for it and decided to see it, and sold it for 200 bucks, and it took a couple weeks to move even at the "fast sale" price.I've got first hand experience with both brands. SO is an Evolution. Kuiu was a first Gen. I was attracted to the light weight and load carrying capabilities. Kuiu was light and comfortable. I only packed game out once with the Kuiu but have put on many miles of training hikes including a 20 mile boyscout hike/camp weighted at 40+ lbs.
I believe my Evolution is the first generation of SO packs. I've also got miles of use on this pack and multiple animals.
My experience is that the SO carries loads much better. My training sandbag is about 60# and the Evolution handles that without problem. I was always afraid I would break the carbon fiber legs on the first gen Kuiu when I set the pack down while hauling the bag. They have since removed those legs (for lack of a better term) in subsequent designs. That brings me to the biggest problem with the Kuiu.
The OP didn't address durability of the packs. That is the biggest difference between the two. The Kuiu's light weight came at the expense of durability. Mine was falling apart before I had a year on it.
To their credit Kuiu replaced it with a Gen2. When I called to arrange the replacement, the lady at the other end of the phone said they were targeted toward guides, and intended to last for only a season of guiding. Not good for me.
My SO Evo has become my #1 pack, and while it's dirty, and it's developed a squeek that I have to trouble shoot, it's trucked along without significant problem. The only call I made to SeekOutside was to compliment them on the material lining their inner hipbelts. The inner lining is durable and grips well to hold the load up in place. Guy I spoke to said he helped design the pack and they were especially happy to have found that material.