[TheFluff]

HE-AAC compresses insanely good but it also sounds like shit, don't use it unless you're extremely pressed for space or the original audio is shit already. For normal sized (120-230 MB) TV-rips I wouldn't advise going below Vorbis q 3 or 80kbps AAC, it's not really worth going much lower, since you don't save all that much bitrate but the quality degrades rather sharply.

For 5.1 AAC you want at least 250 kbps or so.

As for popularity, AAC seems slightly more popular, especially since all the MP4 releases use it.

Edit: if you use vorbis q2 for your music your ears are broken. :|