Hear is my experience for what it's worth. I bought a used Sig P226 two-tone and it came with a Kydex holster and it scratched the frame up pretty bad.
I have not used another Kydex since, I have a leather holster and a couple of nylon ones and have had no issues with any of them marking up the finishes on the guns used in them.
To respond to Echo's post, when would you get dirt in holster, outside of dropping your gun or rolling around in the dirt? For cleaning you can through the nylon holster in the wash.