Windows, but I could happily use either. In fact, I'd really like to do what my friend did. She bought a Mac and split the hard drive. She's half Mac for every day stuff and half Windows so she can run video games.
But I'm happy with my beast. His name is Rodney. He's got a 17" screen, 500GB hard drive, 14GB RAM, and is awesome. And he was only $600, which is really cheap for a computer this good. And, he's an HP.