Windows and their machines have the advantage of longevity right from the start. They were the first widely used PCs and continue to occupy that niche to this day. They have the advantage of familiarity with most of the world’s population from a tender age. That said, Apple has had a meteoric rise since it was founded in 1976 and has come up to provide some stiff competition.
Apple machines that run on the OS X have high-quality support; this is largely due to the professionalism offered by both their hardware and software technicians while Windows machines seem to get much more lax support. Also, although not completely virus free, the Apple machines are significantly less susceptible to malware than their windows counterparts. This could be attributed to the proliferation of Windows machines, but it is still a plus on the Apple side. As far as software diversity goes, Windows beats Apple hands down. Windows has one of the most diverse arrays of end-user software on the planet while Apple still has a long way to go. There are applications exclusive to both, but Windows will spoil you for choice.
As far as hardware, Apple machines are more expensive than Windows but seem to offer more value for money. They last longer and are not as prone to breakdowns as Windows machines. Windows hardware can be customized to customer specifications and is easily upgraded while Apple is rigid and even minor upgrades are expensive. This means for an equivalent price you could get a more powerful Windows machine than an Apple machine. This flexibility in hardware manipulation also means Windows machines offer a far superior gaming experience than Apple for niche customers.
All in all, Windows machines have the upper hand for now because of the head start they got at inception. However, the market is dynamic, and Apple is, and will remain, a force to be reckoned with!