Thanks everyone! I ended up getting both so I thought I’d post a review for anyone else that stumbles across this thread. I got to try both in 90+ degree CA heat (so no humidity).
I liked both shirts a lot, and both kept me cool. I got light colors in each (why??), a nice blue (with pink) and a pastel purple (with black piping, this is my favorite) in the kastel and plain light silver in the coolibar.
The Kastels definitely have a more “slim” fit. The arms were shorter, and the material thinner. They definitely snag more but the fit is a little nicer.
The coolibar - I expected a dark grey and got almost white. It has thicker material on the sides but has a nice zip pocket for cell phones. The material is more sturdy and didn’t snag. More boxy fit. Surprisingly cool for the thicker material. Not really more flattering even though more boxy fit. Longer sleeves. Almost too long and I have super long arms.
I really liked both. I think the coolibar is a little more durable, but the kastel has a little nicer fit. I like the zip pocket in the coolibar, but the trade off is more material.
Anyone have washing suggestions? Thanks!