Most of that mindset is rooted in the history of computer design work. Historically, Macs had much better color matching/correction capabilities. Color correction is extremely important for print work, though it has very little bearing on computer displays (ie: web design).
Currently, pro print developers use Macs because it's what they're used to. Other than comfort level, there is very little that can be done on Macs that can't be done on Windows and vice versa.
Since you're concerned with web design, there really isn't any advantage going one way or the other. For applications that are directly ported between Macs and PC's, I tend to like the Mac versions better. Things like Photoshop typically have a container window for their Windows ports, and the containers really drive me nuts. I much prefer floating palletes that just sit above the desktop. There are reasons for the container windows, but that's really beyond the scope of this post (read: you don't want me ranting about shoddy program designs. )
Also the Mac range have different processors that process way better. Ask any designer and they will tell you, that they switch between programs alot, and this slows the processor down. I usually work between 3-4 design programs, then we have all the communication software like AIM, MSN, Outlook, Yahoo Messenger etc.