AAC is generally a really crappy codec, MP3 is so mature that is can easily produce similar quality nowadays, to get a real improvement you should still use OGG Vorbis Audio, it's a real free format and it has superior sound quality. He may not do wonders at low bitrates like HE-AAC but for general material it's the best codec to choose.
Unfortunately OGG doesn't support mixing channels together for multichannel audio, thus you need higher bitrates for 6ch audio. However AAC sounds pretty much crappy on a good 5.1 sound system with almost any bitrate, AC3 or DTS are still far superior to AAC. I think the problem is the prefiltering on playback that AAC requires, it uses basically the same trick MP3Pro introduced.
OGG Vorbis should also work in MP4 so you can actually keep using and just ditch AAC. AAC is just hyped because it's prolly the next industry standard for audio and the thing you call MPEG-4 audio, however, it sucks. Industry likes it as it's just another codec that they can make lots of cash with by licensing and of course it supports DRM, just the same reason they like the mp4 container, as you can also enhance it with DRM.
Anyone still using ogm must be an idiot
