Mac.

Background to this choice: I'm a computer scientist these days. I do research on computers and research about computers and I teach computing. I use a Mac. Actually, I use two - an iBook at home and an iMac in work. I used to have a G5 in my last job and I loved it. I occasionally use Windows but I find it overinflated and unnecessarily complex - and a lot more unreliable.

I chose the Mac because it is based on Unix which I much prefer as an operating system. I have far less control when using Windows. I like to be able to get complete control over my machine and I can do that with Unix. I have previously used Linux on a PC and it was okay (I believe Ubuntu is touted as being very user-friendly) but often Open Source software isn't sufficiently documented for my liking.

Nowadays the Mac has much of the software PC users like, such as Office for Mac (so you get PowerPoint, Word, etc). The one thing I miss from Windows is TextPad, but the trade off is that so many other things are easier for me on a Mac. I use Thunderbird for my email, but many of my colleagues (in a university Computing Department) swear by the .Mac email software.

Security-wise, Macs are way out in front. They aren't foolproof, but there are only very limited ways of infecting a Mac. PCs are great big virusy stores.

Macs are brilliantly designed - great user interface design that just requires familiarising yourself with it, then it becomes intuitive. Apple pay a lot of attention to usability and user experience.

(The best thing about my new iMac in work? I have access to everyone else's shared music library on the network - completely legally.)