Do you want to play games?
If you do, don't.
If you don't, do.
'nuff said.
Well, not quite: A Mac will, in general, give you better performance, longevity and utility at (arguably) some expenditure of adaptability, plus an almost complete loss of the ability to play any games that haven't been specifically port for a Mac release.
the other major thing to recommend Macs would be that your internet security concerns will be substantially reduced. Bill Gates and Microsoft are not popular amongst real nerds, so Windows and its related systems are choice (and larger and thus easier to hit) targets for hackers and virus programmers. OSX, for example, is, on the other hand, somewhat more secure, and also somewhat more popular, so the number of hacking and viral threats faced by a mac owner are much less than those faced by a PC owner.
Games are the major deciding factor, though - you'll find that macs, for the most part, run in the lower end of the modern performance spectrum (round about 1Ghz, in my experience, for an affordable model) but that's largely because they aren't geared for games, which are much more demanding on the hardware.
I'm a gamer, thus I use a PC. I count myself lucky to have a fairly stable rig (in that it only throws up a minor glitch once every 36 hours or so, and only two of the games I own inexplicably refuse to work on it). Gaming is my main interest and, hopefully, will shortly become the foundation of my career. If games aren't really your thing, or are just a casual interest, then take my advice and get a Mac and an XBOX 360.
that really is 'nuff said.
