complete information security discussion

What is your understanding of operating systems vulnerabilities and hardening practices?
How should one go about in hardening operating systems and applications, such as a Web Sever? (For hardening of operating systems, you should choose one operating system. For example, you could choose, Windows, Unix, Mac, etc).
Do you think one system is really better than the others or, it is personal preference? Explain your answer.
What are some reasons why an organization would choose one operating system over another? Explain in terms of systems security.