In terms of software, there's really no appreciable difference. And frankly, the tools you use make no difference in your design ability. I can design just as well on a Mac as I can on a PC.
In terms of workflow, however, the Mac wins hands-down. The Mac interface has lots of little features that will save you a surprising amount of time in the long run allowing you to complete more projects and potentially make more money.
The Mac has advantages such as quickview, exposé, and the way it handles file windows overall.
Quickview - allows you to highlight a file in the finder, hit spacebar, and see a preview of the file. This works mostly on images (including PSD files), but also text files (including Microsoft Office docs). Very handy when looking for a specific image in an images folder...and much quicker than having to open each one in a program just to view it.
Exposé - allows you to see and navigate to all open windows. This is much better than seeing a bunch of yellow folders minimized in the taskbar. OS X even will tell you the name of the window when you hover your mouse over it.
Interface - Windows puts all file windows within a root window. So all of your InDeisgn and Illustrator docs are contained within one large window. OS X does not have a root window. So you can conceivably put a Photoshop window on top of an InDesign window. This makes transferring files from app to app significantly easier.
Again - your work will not look any different on a Mac or PC. If you're a good designer, you can create award winning work in Microsoft Word and MS Paint (although I can't say I can recommend that combination). OS X is advantageous for users who have multiple programs open at once, and are constantly switching between them. Windows is just not designed for that.