It's somewhat ironic that your view is so dialectic (that is, binary), when in reality both now effectively use the same hardware, thus making the real question, what branding/operating system do you prefer?
In this regard, depending on audience, there are a multitude of potential answers.
First, it is a common misconception that PC == Microsoft Operating System. 'PC' more traditionally expands to 'IBM Compatible PC', but literally just means Personal Computer (That is a computer used by one person, as opposed to some kind of supercomputer which would in modern times perform some kind of time-sharing division of resources.)
Second, since you are talking about a Personal Computer choice, we can refine the question to be: What kind of operating system do you (an average consumer) use if given a choice?
With multiple choice answers that include: Apple OS X (Leopard or older version), MS Vista, MS XP, Ubuntu (xubuntu/kubuntu/etc too) Linux, Redhat (CENT OS/etc) Linux, SUSE Linux, Other.
However such a question does not often lead to good data in the first place, it assumes that the subjects have been exposed to the various choices (Of those choices I've actually used Neither of Vista or Leopard, though of the two I'd pick Leopard if it ran on stock PC parts without hacking it... (it does if you hack it, or if not, could shortly, just like the versions before it.))
A less misleading approach is to ask what uses in an average week/month that computer would see. Depending on the activities performed one operating system may have an advantage over the others. Though the single biggest advantage to come to mind is actually one of consumer lock in compatibility. Because it has the largest market share Microsoft Windows (XP specifically, but Vista may be a secondary target) is the first target for most applications. However that platform has poor compatibility with other platforms (the work involved in porting between a generic BSD or Linux distribution and OS X is far less) it is often the only target for development. Which encourages users to pick Windows as their OS, thus closing the feedback loop and raising an artificial barrier to real competition. (It's everyone's fault really... any one link in the chain broken and it wouldn't work. However the consumers and developers are the strongest portions at this point. The path of least logical resistance would be MS supporting another API that works with other more open standards. However there is actually every business reason for them to NOT do that, because it would break their virtual monopoly.)
