To some extent, you can also mix and match window managers with desktops -- for example, I prefer to use Sawfish with GNOME rather than the default Metacity, because it is faster and does not show ghostly outlines of windows as they are being minimized on slower systems. However, you should check compatibility before changing the default window managers of either KDE or GNOME.
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Other users prefer minimalist tiled desktops, such as Ion or ratpoison, which are designed to be used from the keyboard rather than via the mouse, and often require configuration before they can be used.
In the last year or so, too, desktops with 3D effects like Beryl and Compiz are attracting the interest of those with the RAM and the processors to use them. However their instability still makes them little more than toys -- despite the race among distros to be the first to ship with one of these desktops as the default.
Amid such a variety of choices, how do you decide which desktop to use? Many people will probably want a minimalist install of both GNOME or KDE, simply because of the number of programs written for them. Beyond that, however, your choice is a matter of priorities. If you want the comprehensive sort of support you get from desktops on other operating systems, you may be content to stick with GNOME or KDE, or perhaps Xfce. If you feel more adventurous, then you might try the Rox Desktop or Symphony OS, while, if system speed is your concern -- either because you appreciate efficiency or have an older system -- then a window manager or a tiled desktop is probably your best choice.
Alternatively, if you want to investigate them thoroughly, why not install as many as your distro includes? In return for an investment of perhaps ten gigabytes of hard drive space, you can investigate all of them at your leisure, choosing the one you want from the login screen. After all, that's one of the advantages of free software -- you can test drive programs to your own content until you find exactly the one that suits you.