Well I have 4 KUIU pieces and a lot of Sitka pieces. My KUIU pieces (tiburon and superdown) are nice, but they are not super durable. The pieces are lightweight and and work well except for the Tiburon zip tee. I had to have the forearms altered by a seamstress because they were way too tight. Keep the superdown away from any kind of brush with thorns. They are a great mid-layer, but won't hold up long as an outer layer.
I've got a ton of of Sitka stuff. This stuff is totally battle tested and has been extremely durable. The only product I haven't been thrilled about is my Flash 32 pack. I've got ascent pants, timberline pants, mountain pants, traverse zip tee, Jetstream jacket, Jetstream light vest, Kelvin vest, some different gloves, hats, etc. I think if Sitka made stuff that wasn't good, I would have encountered it by now. I think the biggest advantages of Sitka are inventory, ability to try them on before you buy, and more specific sizing (I.e. Actual waist sizing rather than m,l,xl). The KUIU stuff is ultra light weight, and I think the superdown is a product that really is unique.