Irrelavent. OSX is a competitor to windows and apple has marketed it as such (I'm a Mac/I'm a pc commercial) and they were specifically targeting Vista. I don't see how not being able to buy it and put it on a machine of your choice suddenly makes them completely different and non-competing products.
Actually it is quite relevant, the target audience does matter since you're looking at direct competition. If I owned a Dell machine, had a bad experience with Windows, saw the OSX ad, can I simply spend $30 and get OSX on my machine legally (in accordance to the EULA), no. So in terms of OS choice, OSX doesn't apply since I can't just buy the OS, I'd need to spend at least for the cost of a Mac mini (costing hundreds) or thousands for any other Mac computer.