Windows. I haven't tried installing Linux yet because I wanted to make sure it works with the factory OS first. Other people have issues with GPU drivers in Linux. There's also the issue of the removal of the S3 sleep option in the uefi.

Yeah there are at least two known issues with it under Linux:

Display flickering if you have the low power LCD: https://forums.lenovo.com/t5/Fedora/Random-screen-flickering...

Microphone is very quiet: https://forums.lenovo.com/t5/Other-Linux-Discussions/T14s-Ge...