I switched from PCs to Macs three years ago. I have been in the development world, mostly, and was doing programming on PCs for over a decade.
That said, now I wished I would have switched sooner. But not for web development reasons.
(warning: generalities follow)
PCs are wonderful for gaming and native programming. They are easily customized (my PC desktop I have here is custom-built) and quite flexible hardware-wise. They tend to work better with backwards compatibility. There's also a vastly higher number of software products out there. And, they're cheap.
However, they are awful for security; easily hacked, and loose network security. Viruses, trojans and spam runs rampant on PCs. They are a nightmare for Audio/Media work and anything requiring 'drivers' (I do sound engineering/recording and would never, ever do anything on a PC). Vista is, well, beyond comment. They are often twitchy with USB devices.
Macs are excellently built; their design is sleek, the branding top-notch, and the OS is probably the best OS on the market (yes, including Linux, because Linux OSes tend to not be user-friendly, and that's a category worth considering). Macs work with nearly anything plug-n-play, instantly. There are virtually no viruses to worry about. Their built-in open source technologies (although the PHP4 version still makes me shake my head) are steps in the right direction. They have properly built network interfaces with sound sharing principles. And no one's said...you can run a PC on a Mac. Uh, trump card.
(And Cocoa is an amazing programming language.)
Macs, however, are expensive. Customizing the hardware is, well, nearly impossible. OS X isn't informative enough when it crashes (although that, admittedly, is rare). Gaming on them is foolish. Software, until this past 2-3 years really, was sparse. And good luck if you dont have a warranty.
That said, there is virtually no difference for Web Developers. All the necessary tools for webdev found on the PC can be found on the Mac, and vice versa. Each performs well, XAMPP or MAMP. All the browsers (minus Chrome, soon to change) can be found. Photoshop can be found on either.
So I feel that the Mac/PC argument for specifically web dev/designers is, well, silly. Now if you're an audio engineer, then we can talk...
