Windows has a better security ecosystem. On the other hand, Apple tends to have more expensive hardware and has a smaller market share, so it attracts fewer malware writers........
.....Technologically speaking, PCs are a little more secure than Macs. Macs have a larger attack surface out of the box (Flash, Java, support for a million file formats, etc.) and lack some anti-exploitation technologies found in PCs like full ASLR [Address Space Layout Randomization]. This means Macs have more vulnerabilities and it's easier to turn a vulnerability into an exploit on the platform. Despite the fact it is less secure, paradoxically, Macs are actually safer to use for most people. This is because there simply isn't much risk of being exploited or installing malware......
...If you believe the hype and the flashy commercials the answer would be Mac. But if you take a look at the two platforms, and the mindsets of the companies behind them then the PC wins hands down. If you compare Windows 7 to Snow Leopard, then the simple winner is Windows 7. Microsoft brought in teams of security professionals to look at their code and find problems leading to a more secure product while Apple is often criticized for ignoring issues....
...Mac malware is becoming more common, is in-the-wild, and is financially motivated...