I think you could make Windows just as stable and reliable as OSX. All you would have to do is start with a known quantity hardware platform and optimize the OS for that specific hardware.
It's simply a question of whether you're willing to pay the extra cost for the MAC and live with the limitations of having to wait for them to catch up with new hardware technology before you can use it.
On average, the MAC will be more reliable and more stable than the PC, but there is a cost and there's more to it than just the difference in purchase price.
That one's been going both ways lately. Apple has been notorious for being late on video cards, but they seem to be getting the latest Intel CPUs and chipsets before anyone else.