There are many things tech'y people argue about online, from vim vs emacs, Python vs Ruby, imperative vs functional languages. The list is nearly endless.
One of the most common is "Which is the most secure OS". Typically it goes that Windows is the most ridiculously insecure OS, and Linux the far more secure. Mac OS X users then point out that they don't get viruses. Then a Windows user points out that he's run Windows for years without getting any viruses, and that there are lots of exploits for Linux applications.
These arguments are all pretty much right. Windows is insecure. Linux is more secure. Mac OS X doesn't get many viruses.. But you can make Windows secure, and Linux insecure..