I've been using First Lite while hunting in Wyoming, Colorado and Oklahoma for the past 5 years or so. I have their merino wool base layers (Llano, Chama and Allegheny), two pairs of Kanab pants (Kanab and Kanab 2.0 - replaced by the Obisidian) and a bunch of other stuff. I just bought their Corrugate Guide Pant (not merino) last week but haven't used them yet. I bought it all through ProMotive (or whatever they're calling it this week).
I love their merino wool. I layer it with my issued waffle top and bottom for added warmth when I'm going to be stationary. The base layers excel during active hunting/hiking/snowshoeing and they're all I needed under a pair of my Kanab pants even when temps were below freezing.
Now that I'm living in Kansas for the year, I'm looking at adding some of their outer layers for stand hunts but I don't have any experience with them.
As far as sizing goes, I wear the same size in all of their gear that I wear in my normal clothing (I'm 170lbs and 5'9" so I wear a Medium in pretty much everything). I have found that the new Corrugate pants are a little more snug below the knee but I have large legs so that could be a factor.
Durability hasn't been an issue over the past few years. I've worn some small holes into the base layers but with the amount of use they've gotten I'm not surprised. I took a stick to the shin and tore a hole in my Kanab pants but after a call to First Lite they sent me some material to patch it up with.
If you want solid colors, I have black, dry earth and conifer. The conifer is a really nice OD green shade. The dry earth is basically coyote.
I haven't used Kuiu or Sitka to compare but with the ProMotive discount I don't see any reason to change things up. Everything that I've read on some of the outdoor/hunting sites make me think First Lite is the way to go for merino base layers.
Let me know if you have any other specific questions that I can help answer.