When should I use AAC rather than MPEG-2 BC?

Both provide 5-channel audio coding capability, however AAC provides a factor of two better audio compression relative to MPEG-2 BC, and is appropriate in all situations in which backward compatibility is not required or can be accomplished with simulcast. An MPEG-1 two channel decoder can decode an MPEG-2 BC 5-channel bitstream. AAC has no such backward compatibility requirement and, for 5-channel audio signals, has been shown in MPEG formal listening tests to provide slightly better audio quality at 320 kb/s than MPEG-2 BC can provide at 640 kb/s.