They're barely even competitors, IMO. Apple's where you go to pay a premium for the closest thing to a just-works, well-integrated OS & device ecosystem that the market provides, for home users or individual pros (or small businesses that don't do much in the way of "fleet management").
Microsoft (rather, some manufacturer shipping MS-bearing machines) is where you go if you don't want, for whatever reason, to pay a premium for that, or you need the best business-oriented integration & management the market provides.
That covers about 99% of all personal and business computer users, between those two—the remaining sliver is mostly Linux and the BSDs. But there's not a ton of market overlap between them, I don't think.