SW Minifigs Notes
At first keychains were added because they differed from the non-keychain figures for obvious reasons. But the last few years they often differ in color and/or print. For example: more recent keychains do not have a two sided head and Luke's keychain 853472 (picture on the right) has never been released as a non-keychain figure.
MiniFigs.nl - see the minifigure decision tree for more information