I'm not even sure "Windows vs. OS X" is the best analogy. Nikon and Canon are so close in quality, features, and price that choosing between them really comes down to timing. Once you choose one over the other, though, and you sink hundreds or thousands of dollars into lenses, flashes, remotes, etc., you're a customer for life.
I bought the 10D when it first came out and it was generally considered a better value for the features at that time compared to the most comparable Nikon. When I decided to upgrade to the 1DIIN, I already had a flash, some lenses, etc. for the Canon system ... and Nikon didn't have a camera that could take 8.3fps, which was the major selling point of the 1DIIN for me in the first place.
Then I bought a 16-35/2.8
L, 24-105/4
L IS, 70-200/2.8
L IS ... there is no way I could switch now even if Nikon came out with "the world's best camera." Which they haven't.
So when it came time to buy my wife a new dSLR, I didn't even look at Nikon offerings. No point recreating the wheel with all new lenses, etc.