Audacity is a lot simpler from what I can tell. It is meant to record and edit audio without doing anything fancy. Ardour on the other hand has lots of additional functionality, such as a fully featured mixer with plug in support and the recently added clip launcher. There is probably a lot more to say, but if you are serious about making music, definitely choose Ardour. If you just want to record something quick, or something simple like a voice recording, Audacity can get the job done.