First of all... don't convert. You should use what works best for you. We don't need more Mac users just to have more more Mac users.
As for what makes a Mac different from a PC in the area of graphic design... it comes down to graphic designers being one of the few groups of users who truly need to multitask. I'm talking about user multitasking, not computer multitasking... there is a difference.
In Windows, most applications are designed to take over the interface of the system (rooted apps within full screen windows). When working on an image in Photoshop in Windows, you have the image window inside a root window which takes over the screen.
Now this is fine if the only app you use is Photoshop, but for most graphic designers, Photoshop is one of many tools being used, and when that root window takes over the screen, they are isolated from the other apps that they were working with.
On Macs, applications take up only what is actually needed by them. In this way you can see other apps in the background and even the desktop, which many people use for holding items that they will end up dropping into a piece of work. You can't drag-n-drop from the desktop (or any other app) if you can't even see the desktop. The ability to drag-n-drop is a major advantage in work flow for users who multitask.
Personally, I think Windows is a great platform for secretaries, gamers and the like. People who only do one thing and don't want to be distracted while doing that thing.
That having been said... you should use what you like. If you don't know what that is, I highly doubt anyone else can help you figure it out.