I think both are okay for webdesign. It is more important to consider in on how you are capable of using such. If you are more productive with PC then go for PC. On the other hand, if you are friendly with MAC then go for MAC. What matters most is the specifications of your system. The newer and the higher the specs are, the better the system is.
I agree with Fleigermail[FONT='segoe ui', helvetica, 'lucida grande', lucida, tahoma, arial, geneva, verdana, sans-serif].[/FONT]
If you want reliability, a Mac wines every time. If you want cheap and cheerful the a PC is king. On a serious note, both do the same job just as well as each other. Macs will hold there value better, so in the long term offer better value for money.