Main use of AAC/MP4 Audio are
1) Quality audio at lower bitrates (Similiar to 320kbps MP3 Format)
2) Improved audio quality at higher bitrates (Better than 320kbps MP3 Format)
For encoding the best option is nero 6 or 7 ultra edition
If you are planning to hear MP4 in mobile phones nero is'nt a good option,go for Imtoo 3gp converter (
http://www.imtoo.com/3gp-video-converter.html)