The largest reason there aren't many known vulnerabilities and not a large amount of malware for OS X...
...is more or less irrelevant, unless you personally have pissed off some hacker kid in the last couple of weeks.

We've always had at least one Mac in my house as I was growing up, which puts me in a somewhat biased position, but also puts me in a position of someone who has genuinely spent extensive time with both operating systems.
I'm also computer-literate (have dabbled in programming for a number of years, entered a couple of small competitions, and study computer science at uni, here). Obviously it's just one man's opinion, but I genuinely feel that Mac OS X is a nicer operating system to work with, on a day-to-day basis. I easily get more frustrated sitting at a windows box trying to do things than I do with a Mac. (I should state here that I have not yet played with Windows 7, the latest Windows Install I have access to is Vista. And I wish it were XP.)
Macs certainly aren't marketed to the "go under the bonnet, fix it yourself" group, and that means they've made what I feel is a much more user-friendly computer. The irony is of course, if you want to go "under the bonnet" in a software sense, I would argue it's considerably easier tangling with Mac OS X's darwin kernel than the beast which is Windows.

But at the end of the day your average user probably doesn't care too much about the above.

Macs are purdy, bitches in cafés will get on your dick, and if for some reason you dislike Mac OS X (which would surprise me greatly, if you spend the requisite time realising it's not Windows) then you really can just install Windows or Linux on your Mac and have both.

Buy a mac, imo. If you want further reasons from a completely impartial and unbiased source:

http://www.apple.com/getamac/whymac/

In summation: Buy a mac, get bitches.