I would say definitely Linux, but then I am a Linux user. Doubtless you'd get a similair response from a Windows user. I used to have Windows and Linux on the same machine, but I noticed several months had gone by without me using Windows and couldn't think of any real reason for me using it again. Windows was on the larger of my two HDD's and I figured there was no point wasting it on something I never use.
You can have both operating systems on the same hard drive but I'm pretty clueless with computers and have no idea how to do it properly. If you do want to use Linux, I'd highly recommend using Suse. It's nice n' idiot proof (which was perfect for me when I was starting out with it), and has a feature (called "YaST") which finds and installs things for you which can be a pain when you know what you're doing and a nightmare when you don't. Dave ("the 'tache") Harrison tried out the new version of Mandrake (another popular distribution of Linux) which has recently been renamed to Mandriva and it seems it doesn't have the YaST feature (which meant things were a bit of a hassle to install).
There's also some "short" guides I've written (initially intended for Bob but would be helpful to anyone) on getting started with Suse to get you set up with up-to-date versions of Apache, PHP, MySQL, and OpenSSL as fast as possible with the minimum of fuss.