[fysiks]

HLDS is optimized for Windows but typically on Linux there fewer other applications running so the load on the processor is less.

HLDS and games that use it are ancient and any modern OS will run it just fine. The best way to go about it is to use the one that you are most familiar with.

Around here, I most often see CentOS and Ubuntu mentioned for Linux. There are many discussions about this topic so if you don't like my advice, you can search for them.