MAC OS X *is* UNIX, so you get all those server tools built-in like SSH and telnet. S/w like Fetch for ftp and Dreamweaver for site management are mature.
I run a paysiste
www.scifierotic.com and haven't found I am missing anything crucial, yet.
Macs display colors differently than PCs, so if you are doing image editing you need to know this. And of course every browser on the planet displays pages differently.
SWISH is PC only. Its the only thing I've wanted so far and couldn't get.
Yeah there are about a zillion more s/w products for PCs, but half of those are games. Pretty much everything you *need* is available for MAC, because that is what the smart people use.
MACs are faster and more reliable. Don't get viruses. Network better. Handle file managment windows better. Easier to back up. And...they make you cum 30% harder. -)
PCs are built on the concept of 'good enough.' And for 95% percent of the world, they are.