Well, once upon a time using a PC meant this was your environment, whereas using the Mac let you work with this.
The IBM / MS-DOS computers were ubiquitous because of the business-world clout of IBM. Think what you will of the modern Microsoft operating systems, but believe me, in the beginning people did not initially adopt the IBM PC out of appreciation for the operating system. The Mac came later, and those of us who saw it thought it was as much an improvement over DOS as modern indoor hot & cold running water is over a hand pump in the yard. I admit it — we were arrogant and evangelical as all get-out. “You’re using a PC? Why don’t you get a Mac!?” But, really, we had reason. The only thing the PC had going for it was the large installed base and the things that come from that (lots of software, lots of compatible hardware). And the hostility of PC users to the Mac — “It’s a #$@@ toy, that silly ‘mouse’ thing, ooh you click the pretty pictures to run pwogwams that’s so cute, it’s a toy” —just fueled us up that much more.
The Microsoft operating system has come a long long way, and so has the PC hardware. Y’all don’t use text-based screens any more, you use a screen like ours, with color values for each pixel. You’ve got a mouse. You switched to the same 3.5" diskette that we used (until we quit using floppies a few years ago). Your software took on standardized motifs — menus at the top, usually File on the left, then Edit, with common keyboard commands such as Z for undo, X for cut, C for copy, V for paste, P for print, O for open, and so on, just like the Mac. Your OS can support multiple monitors now, and supports plug and play addition of new hardware.
(To be sure, there were many hardware developments that appeared on the PC first or at least no later than on the Mac, and the Mac platform either followed suit or hopped onboard along with the PC — USB, the VGA monitor port, function keys (Fkeys), PC (PCMCIA) cards, AirPort (WiFi), support for multi-button mice, etc — but none of them were there in the original PC. No convention of the old-world PC was adopted by the Mac platform as The Way It’s Done, the switchovers were all in the other direction because the Mac really was ahead of its time.)
A lot of the rest of it is just inertia and old rivalry. If the two platforms somehow popped into existence only now, without the rivalry history and starting from square one as New Things (no installed base, no software offerings yet, no viruses or malware exploits either), I don’t think the same fervent evangelical thing would develop, not with that kind of intensity. I think there’d be much more mutual respect, although there’d be personal preferences and whatnot for one or the other.