Exactly why is F/OSS better?
It's subject to peer review. Some of the best programmers in the world have access to, and readily submit, code for F/OSS projects (not to say that EVERY F/OSS project is superior mind you).
Look at why hackers use it. Aside from their ability to heavily modify their system, they're also extremely paranoid. I know plenty of hackers that contribute code and readily fix problems in F/OSS code because of their own paranoia.
Look at why the DoD and NSA use it. Its laid out like an OS should. ACLs, chrooting, SELinux, all of these help make it much easier to protect their own systems.
Want a really good blast at Microsoft? OpenBSD, its been around since 1994, there have only ever been 2 exploits off of the default config, and one of them was for a legacy version. Heck OpenBSD + pf is what the Defcon guys use.
And quite damn honestly, code that's open source has met the firing squad. Hackers CAN see the code and compile it themselves, making it EASIER to find exploits, but yet Linux is regarded as far more secure just makes me think about how secure Linux REALLY is in comparison to NT. If you could place the NT Source code in the hands of someone competent I'm sure it would be hell for M$ (just when you thought it couldn't get any worse than MS08-067).