I have about a half dozen pieces from both, and just got back from a black bear hunt in N.C. and basically used all the pieces last week.
First, I would say you can't go wrong with either company, they are both phenomenal hunting clothes. Light years ahead of cotton crap I grew up with. That being said, they are definitely different from each other.
The thing that separates kuiu from all the rest (to me) is the weight. The stuff is freaky light. For example, I switched over from the cabelas rain suit to the kuiu and I shaved 2 lbs off my pack. That's almost 10 percent! from just switching manufacturers. I also carry my kuiu puffy layer in my pack, and it kept me in the stand all day because it weighs next to nothing, and I had it with me.
The down side to the kuiu (from what I've read) is the customer service, and their sizes. I've never had a problem with their products, but from my research, it can be tricky trying to return things and/or use any warranty on their products. I've got absolutely no negative personal experiences, just gleaning from the various forums that some customers end up pretty frustrated with kuiu when there is a problem.
The kuiu sizing is kinda weird. I'm 6'2 225lbs with 34w and 32 inseam with a little bit of a belly. Everything in my closet is xl EXCEPT the kuiu. They're the only clothing company in the world where I'm XXL. Not sure why they do this, but they run notoriously small. I've triple checked the sizing charts after I've received stuff, and they don't seem to line up. Something to keep in mind when ordering, ESPECIALLY when ordering off the outlet (no returns or exchanges allowed)
The sitka stuff is fantastic. You can really see how much thought went into their products. Personally, I think the pricing on some their stuff is waaaay out of line, but you can definitely bargain hunt the Sitka stuff.
Also, the complaints on their customer service are pretty few and far between from what I can tell. If that's a priority for you.
I think that first lite, and archteryx are pretty much on the same level, quality wise. But they just don't have a Jason Hairston-type marketing genius behind them, so they don't get the recognition. Just my opinion.
Basically, I wear merino base layer, the sitka mock turtle neck, the sitka ascent pants and jacket, and I'm covered from the low 60s down to the 30s. Below that, i throw on the kuiu puffy from my pack and I'm good down to the teens.
Hope you get something you can use out of my manifesto. I've been lurking for over a year, feels good to contribute on something I have experience with! Lol.
Good luck!
Brb