In testing Linux and Windows operating system security, I've found it's more about HOW YOU IMPLEMENT security -- rather than whether one is superior overall.  Either OS can be implemented in a secure or non-secure fashion. 

Any OS needs the following for the best levels of security:

Best Practices: Security Controls for Operating Systems

1. PLAN your security environment and controls before you install
2. Patch, Patch, Patch (keep it updated)
3. Firewall and Anti-Virus controls
4. Avoid highly privileged user accounts where possible , (e.g., Admin/Root),
5. Turn off unneeded services and privileges
6. Stay informed and take counteraction measures on any evolving threat.
7. Test your OS controls continuously in the corporate environment (using free security tools from Microsoft and other vendors)