I have a mixture of both brands, most of my base and mid layers are Sitka, but find the kuiu outer layer and insulating pieces to fit much better than Sitka, I’m average size and in good shape, the Sitka outer layers just don’t fit as well, tight sleeves etc, not sure why.
my favorite piece is the ambient, love it so much I bought another to wear daily, very warm and super lightweight.
the only exception to the outer layer is the Sitka Dewpoint, very lightweight and it fits nicely, they resized the line not too long ago, it is absolutely comfortable and 100 percent waterproof, I’ve worn the Dewpoint in rain , all day long sitting and glassing and stayed bone dry. I like it much better than my kuiu chugach.
I suppose a lot of it for most people is how the clothing fits you personally. I couldn’t ever have and wear just one brand due to the fit.