Not much difference.

A Windows Media Audio file is slightly smaller, a slightly more compressed file.

Example: A wave file song saved as an MP3 at 128 kbps (Joint Stereo) is 2.17 MB

The same wave file, saved as a WMA is 2.16MB

Many experts agree that for roughly the same size file, WMA just sounds better then an MP3. Indeed, Napster, and many pay music sites are using WMA, because they believe the quality is better, in a smaller file size.

So it's just a matter of choice, although in my personal opinion, WMA gives better quality than MP3 for lower bitrate.

SeeYa,
Larry