Little people know that the audio quality of AAC > MP3 at the same bitrate. You can get AAC files from iTunes with the suffix m4a at the iTunes Store for example. Today I used the application “Spek” to visualize the spectrum of the same rock song with both encodings in comparison. You cannot really notice the difference by hearing (unless you are an audiophile with good equipment and very good ears) but if you want the superior audio quality with smaller file size just choose AAC files. For most people, the top audible frequency is, depending on age, at 18–20 kHz. In the diagrams, you can see that mp3 files cut frequencies at the top.

m4a Spektrogram mp3 Spektrogram
Left m4a, right mp3.