There are a few key things you need to consider when purchasing a new computer and when you're trying to decide between buying a Mac or a PC:
Graphics, Color & Typography
When a designer goes to work, they're focused mostly on graphics, color and typography, so they need a computer that can do the job. Apple has really placed a focus on bring their game up related to colors and fonts, which is particularly important for print designers. Print and colors HAVE to be the same when you print them out as they are on your screen so that your design workflow can be as efficient as possible.
Mac has also presented designers with standardized font and color options, making their display much more predictable and clear. This spells happier designs and simplified workflow.
Both platforms can use most of the popular designing software, like Adobe's Creative Suite, and there are few pieces of new software that won't work on both platforms.
Now, it is a good point that PC's do have a wider amount of gaming software than Mac's do, but for designers this doesn't seem to be a really significant deal. However, photographers and designers who do a lot of image editing seem to favor the Mac platform because the resolution and color contrast are much more true.
Mac's do have a variety of shortcut keys to help with the efficiency of your workflow, resulting in saved time and the ability to multitask. PCs have shortcut keys too, although these keys are different from program to program.
An entry level Windows laptop costs about $300 while a comparable Mac costs about $700.
When it comes down to it, it seems that the choice is based on personal preference. Are you a graphic-demanding designer who wants clarity, true color and ease of function? Are you a fan of gaming that wants the ability to operate nearly every type of software?