A OS is trully only as secure as the idiot behind the keyboard, but hey removing that wild variable. I would swallow my microsoft lovin' tongue and say Mac's for a couple of reasons. Main one being the closed or limited hardware and software that can work with the OS.
I think a person like him would probably be marginally more secure on a Linux system, because he's knowledgeable and comfortable with the options. I'm not sure it would be true for an average user.
Comparing Unix to Mac OS is like comparing a swiss army knife to a butter knife.
Security is a function of your desirability as a target.
Mac's aren't a big target because there aren't many (relatively speaking) and very few, are doing anything worth attacking.
Where are hackers going to spend their time? On Unix and Win based machines which run virtually every where and everything or Macs which are mostly in homes and schools? It's the same reason so few hardware or software companies bother making products for the platform - in many cases, it's just no worth the cost.
Ahh, PC security. The global warming of the computer industry.