Simply put, it is impossible to say that a PC is better for design work than a Mac just as it is impossible to say a Mac is better than a PC at design work. Given the same hardware, the only difference is the operating systems, which also function nearly identical in performance.
In the end, it all comes to personal preference, which has been stated before. There is no wrong or right answer, there is only which you prefer and the reasoning behind it. The reason that I approach Macs and PCs in comparison the way I do is because of the facts behind the differences and similarities in these computers. Really, the only grudge that I can honestly say I hold against Macintosh computers is that they intentionally try to heighten the false rumors and false sense of superiority that they have over PCs when, obviously in fact, is only just a lie. To me, Apple is in a way directly lying to their customers to increase the "awe factor" of their computers to justify their price.
Again, just my opinion. I don't feel that anyone can truly make a justified factual claim that one computer is better than the other system in terms of design (with all hardware and software being equal of course) without said justification being based purely on their preference on use of one operating system over the other. I want to thank Vector Doctor for explaining out reasoning behind preferring one system over another, as it makes sense. Macs do come with a lot of included software that today Windows is still trying to add and integrate with their systems.