George Ou from ZDNET has looked into volunerability statistics of Apple and Microsoft OSs.
"The data is clear, and Apple has a lot more vulnerabilities of every kind ranging from moderately critical to extremely critical. While Windows had some months with more security disclosures, they are more spread out while Apple tends to release mega-advisories with dozens of vulnerabilities at a time. There were seven months where Apple disclosed more a dozen or more highly critical vulnerabilities and August 2005 saw nearly three dozen of them. "
Microsoft on the other hand seems to let some moderately critical and even one highly critical vulnerability go unpatched for more than a year.
More: http://blogs.zdnet.com/Ou/index.php?p=16 5&tag=nl.e622
