Until recently, it was a commonly held belief in the mainstream computing world that Macs are more secure than Windows PCs are.
Then MacDefender happened, and a whole new era began.
Suddenly it became clear not only that Macs have begun to attract the attention of malware creators in a significant way, but also that Apple isn't particularly interested in helping its users out in this new era. Rather than jumping to attention with assistance in a timely manner, the company dragged its feet, denied the problem and stonewalled in every manner possible before finally taking action.
For a company that prides itself on the safety and protection of its tightly closed "walled garden," the arrogance of Apple's reaction was beyond belief.
One Platform Still Standing
Regardless of your opinion of Apple and its approach, the fact is that the computing landscape is irrevocably changed in the wake of MacDefender and--later--MacGuard. While these certainly weren't the first bits of malware to target Macs, they were among the few to succeed so well.
The result is that Mac users can no longer rest easy in the "security," real or perceived, of their walled garden--at least no more than Windows users can on their own platform. If anyone needed further proof that "security by obscurity" doesn't work, this is it.
Two closed desktop operating platforms have now "fallen" to malware, in other words. That leaves the open one still standing: Linux.
Strength in Diversity
I'm not saying that Linux is impervious by any means, or that its security is perfect. No computing environment can ever be perfectly secure.
It is worth noting, however, that Linux still hasn't fallen prey to anything near the level of insecurity that Windows and now Macs both have. In fact, several distinct advantages make security one of Linux's strongest selling points, as I've noted before, including the way privileges are assigned, the openness of the code, and the fact that desktop Linux users still make up a smaller audience.
Before anyone makes the argument that the audience-size advantage will disappear as Linux gets more popular, though, let me also note that Linux is made up of countless different distributions, making it tough to hit too many users at once. Ubuntu, for example, may be staking the biggest claim on the desktop of any distribution to date, but it's still just one of many Linux flavors, making it a lot harder for malware authors to strike big in one hit.
The Open Advantage
Audience size may still be one factor working in Linux's favor, but I think the bigger one is the openness of its code. Even if Apple did care about customer service, the fact remains that no one company can protect you as well as the worldwide community of users can.
Bottom line: Want security? Get Linux in the flavor of your choice.