Gunnar Van Vliet has written an informative article at RecordStoreReview.com comparing the quality of AAC vs. MP3. Although the tests were unscientific, Vliet posted the results of one songs performance compressed at 96, 128, 160 and 192Kbps in both AAC and MP3 formats. He rated each test group for tonal accuracy, imaging/soundstage, naturality, and musicality.
AAC vs. MP3: Tested and Rated
“On the whole, there weren’t any surprises. My 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.”