Google Earth is a well-known 3D geography browser from Google, available for Macintosh, Windows, and (in beta) Linux. World Wind is an open-source 3D geography browser from NASA, available for Windows only (though a cross-platform version based on .NET and Java is planned for late 2006).
Both are sophisticated programs with amazing feature sets, and both have features that the other lacks but would benefit from. However, as similar as they are (or seem to be), Google Earth and World Wind are not entirely comparable because of significantly different focuses, development histories, and development philosophies. Many of us use both programs, depending on what we wish to accomplish. Each program has distinct advantages over the other, and so we will leave it to you to decide which you prefer (if you absolutely must have a favorite).