Rule one of security: Don’t assume that your software is safe. Apple users (not Apple Corporation) make the assumption that their software is secure from viral attacks. Is this true? How do you determine if one system is more secure than the other?
You could ask Apple or Microsoft, or a Linux user group. In reality both corporations and usergroups are going to give you as honest as an answer as they can. None of these corporations are going to cause mistrust with their customers.
If you are a CTO, or CIO, or a student who wants to become one, you may want non-corporate related vulnerability research.
One way would be to do some research, I came up with the http://nvd.nist.gov. I use the vulnerability database to search and compare Windows 7, Mac OS X and Linux. I used the keywords as shown, and took the raw numeric return, included problems caused by the competing operating systems
Windows 7 over the past 3 months: 39 vulnerabilities (Keyword= Windows 7 )
Mac OS X over the past 3 months: 86 vulnerabilities (Keyword= Mac OS X )
Linux OS over the past 3 months: 118 vulnerabilities (Keyword= Linux )
Both systems appear to suffer from buffer overruns, but Mac OS X had more than Windows 7.. This is unfortunate since Apple users (again not the corporation) assume that their system is safe. This does not mean that the security issues are only Apple OS related, or even Microsoft related, rather many of these are external vendors like Real Player or Apple Quick Time (on Windows 7), Blackberry (Both Windows and Apple), Google Chrome (Apple). Naturally, for all three OS’s there are holes that are caused by the OS developers, it appears that stack overruns are still the largest problem.