Elinor Mils from CNET interviewed 32 security experts to answer the question whether Mac is more secure than Windows. You can read the article over here, most security experts seem to agree that while Mac users barely have any malware to worry about due to its smaller userbase, the operating system isn't necessarily more secure than Windows. Here's a snip:
Chris Wysopal, chief technology officer at Veracode: "My wife, kids, and parents all use Macs on my recommendation. I think the Mac is less risky, not more secure. The difference is in the threat environment. An analogy would be an unlocked house in an urban vs. rural environment. Both are insecure. One, the rural, is less risky.
Bugs similar to the flaw in Microsoft Internet Explorer 6 that was exploited at Google in China certainly exist in Mac applications, but attackers don't spend the time required to find them and build attacks using them. This is because it is much more often a PC than a Mac between the attacker and the attacker's target."