I do not believe CA has published any documentation that identifies a preferred server OS platform.
Generally, my experience has been that the DevTest server components compare favorably on all supported OS's. If your company or customer is more comfortable with Windows, use a Windows platform. Similarly, if more comfortable with a Linux flavor, use a Linux install.
If you are running performance & load tests, I would be more concerned about the number of cores per CPU, number of CPUs, and memory available to the DevTest Server components. Obviously, different tuning approaches may apply depending on OS, but I have seen solid performance characteristics on Windows, Linux, and Mac OS's.
Just my two cents.