The trouble with PCs is the operating system. Buy a new PC laptop and you'll be stuck with Microsoft Vista. So between those two choices, definitely Mac.
But personally? I'd go with a Linux system (probably OpenSUSE, though Ubuntu is popular too) and then use NVU for the design work and ActiveState Komodo Edit for the coding and The Gimp for the graphics design (all free software by the way, and all available for Linux, Mac, and Windows)
And by the way you don't even have to take my word for it... OpenSUSE and Ubuntu are both available for free online... just burn the CD and boot... no need to install anything on your hard drive to give it a try. Ubuntu will even mail you the CD for free if you don't feel like downloading/burning.
The Leapord OSX MAC!!!!
omg, i went on one today and i fell in love. i sooooo want one!
Macs, being UNIX systems, run most of the software that the web depends on (like Apache) in the environment that most of the web uses, so they are very good machines for the purpose.
Don't try to get into website design by using Dreamweaver though - its a tool that can help with the process, but it makes a very poor substitute for knowing how to write the code behind web pages.