AAC is far superior than ac3,and it does support 5.1, but ac3 is kept for compatibility with older players.
AAC supports up to 96KHz sampling rates, and upto 48 channels
so, basically AAC is far better than ac3 it is a replacement for standard mp3, but on steroids.

http://en.wikipedia.org/wiki/Advanced_Audio_Coding
cvrle77 Reviewed by cvrle77 on . difference between aac and AC3 what is differnent between bluray aac and Ac3 which is best Rating: 5