In a rather tepid conclusion, and after discussing the usual theoretical issues, the author finds that Mac OS X is more secure than Windows in Myth or Fact? Is Mac OS X Really More Secure than Windows? at informit.com. No sense offending anyone with the truth that millions of Windows PCs are compromised, with a huge hacker community and organized crime drooling at the rewards.
The fact remains that connecting a new PC to the internet without jumping through hoops to install anti-virus software and taking other precautions is electronic suicide, which is not the case with Mac OS X. My view is rather stronger on this issue—see my article PC or Mac—Making a Sensible Choice.