MP3 (MPEG-3)

MP3 is a popular audio compression format used by millions of people worldwide for sharing and storing music on their home computers.
MP3 files use different compression rates, but the more they are compressed, the worse the sound quality. MP3 compression at 128kbps is at a 10:1 ratio of the original CD file, and works out at approx 3-4 MB for a 3 minute track.
WMA is a higher quality (bitrate for bitrate) format than MP3
So if you were to take the same song and encode it on MP3 and WMA at the same bitrate, then the WMA file should sound better.