http://www.macdailynews.com/comments.php?id=P1036_0_1_0

On the whole, there weren't any surprises. Van Vliet's observations echo what most people have said about AAC vs. MP3. AAC is higher quality at the same bit rate, so you can use a smaller file to achieve the same quality as MP3 which is a good thing for portable and computer users. Ultimately, both formats still sound pretty bad in their practical ranges compared to CD