VLC will play damn near anything; Flip4Mac only allows you to play WMV-wrapped video files. Since Flip4Mac is a QT component it allows you to use QT to play WMV videos whereas VLC is its own separate application. If you want to keep QT but increase the types of videos you can play I would suggest checking out
Perian. Perian is a set of QT components that significantly increase it's compatibility with other video codecs (still doesn't beat VLC, though).