I can't seem to be able to take a decision based on my ressearches, so I went on creative.com.
On one page (or actually Flash movie), it says mp3 is better than wma, which I agree with based on the audio sample they provide. However, they won't say anything about the bitrate they used.
On another page, they say wma is just as good as mp3, while taking less space.
How enlightening!

