If it is an Intel-based computer running a derivative of the IBM PC Architecture, it's a PC. If it's an ARM system, an RS-6000, a Motorola 68000, or some other architecture, then it's not a PC.
As I said, the term is a Metonymy: the only computer that can legitimately use the term "PC" Is IBM's Personal Computer line, which ended with the PC-AT. (After that, IBM called them the PS/2.) Calling all single-user computers "Personal Computers" is correct, as the term was in use before IBM Introduced the model 5150. However, a "PC" is not just a Personal Computer. A PC is an IBM Compatible Personal Computer, distinct from from other computing platforms, such as Mac OS, PowerPC, Amiga, Atari, Commodore, CP/M, or other platforms that were common when real IBM PC's could still be purchased.
When you are referring to desktop computers in general, using the term "PC" may be acceptable, but when making a statement like "Do you prefer Mac or PC?" PC clearly refers to a Windows-based computer descended from IBM PC architecture.
If you don't believe me, I ask you to answer this question: What part did John Jodgman play in the Apple ads? Was it "Windows"? Was it "Intel BIOS Architecture Computer?"
It was PC.
So in its own advertising, Apple Computers clearly states that the Mac is not a PC.
Go ahead and try to call Mac a PC if you want... but you're not longer arguing with me. You're arguing with Apple itself.