It's pretty simple. What apps do you use and are planning to use with your new purchase? Most common apps would be Microsoft Office, browsers and graphics/multimedia which are available on both platforms.

If you are comparing OS alone, Mac OS X is elegant, mature and known for its ease of use. And no spyware/adware and malicious viruses. However, there is a learning curve as you will need a bit of playing around to know how it works as it definitely differs from Windows and Linux. But the Mac community is helpful (esp the boys at MyMUG) and are always willing to advise and help a newbie out. Don't be afraid to ask.

Personally, I'd go Mac, but hey, sit down and think about what you need the Mac/PC for and then you'll get the answer you need