Don't fall into the brand game. A computer's a computer. It's the Kernel and OS that makes the computer run that most people get confused about. An Apple's no different from a Dell which is no different from a HP, ETC. It's the Operating System that matter (and of course the hardware). With that being said, what are you using your computer for? If you're a gamer stick it Windows OS. If you're into music or computer animation stick with a Mac. if you're into programming and want to really know the ends and out of a computer OS go with a Linux distro like Ubuntu 10.10. I use Ubuntu (Open Source) 100% on my desktop and laptop. I could never go back to proprietary OS. If I was to buy an Apple computer, I'd still just install Ubuntu. It's free and quick. And if you really want control over your computer try Arch Linux.
It really does matter what you're using the computer for.
ALL OS have their ups and downs.