I've never been one of the people who bought the idea that Windows was less secure than Linux or vice versa.

Any networked operating system is going to have security holes that need to be patched. Windows was a big fat target with more hackers pounding at it. So it had a lot of published exploits. Microsoft has started to get their act together and is getting better about security holes. So it makes sense that Linux is getting more targeted now.

When you're asking the question "how secure is it?", the argument between open-source and proprietary security models is pointless.

The real questions are... How does one take care of the inevitable security problems which will crop up in both OSes? How much work must you do to harden the OS when you first install it? Does the OS come installed with a bunch of unsecure defaults? How easy is it to remove unsecure processes from the box? How easy is it to detect intrusion when it happens? How quickly can you patch the OS when a hole is discovered? How easy is it to track the hacker's damage and repair it?