Airport Extreme + USB Drive vs Time Capsule
According to Apple, Airport Extreme with attached disk is not supported for Time Machine. It might work, sometimes, but with a backup system, people usually want more than "might work sometimes".

Sources:


http://support.apple.com/kb/HT2038
http://docs.info.apple.com/article.html?path=Mac/10.6/en/15139.html
http://docs.info.apple.com/article.html?path=Mac/10.7/en/mh15139.html