For Windows, I like multiple monitors, since it allows you to maximize a window in each monitor.<p>On a Mac, for the most part apps don't take over a whole monitor without a bit of effort, so one monitor is probably the winner.<p>There's also a Windows shareware utility called SplitView that divides up one large monitor into a couple of virtual screens, so you can have the benefits of side-by-side maximized apps without giving up full-screen gaming.