Well there isn't much in it between Vorbis and AAC now, some cases Vorbis edges ahead, others AAC is better.
Though in true Hydrogenaudio style, I should say ABX OR GTFO. It's kind of a shame that LC-AAC has become the defacto standard for AAC, because if Nero were to implement LTP into their free encoder, I'm sure it would give Vorbis a very hard time (since it's already holding it's own).
A shame, but to some extent it's the community that makes things the way they are.
http://www.soundexpert.info/
Some of these are a bit old, but perhaps will serve some insight. It suggests that AAC has (or had) the upper hand at 105/128kbps, with Vorbis pulling back at 192kbps. Not surprisingly, at the low end (64kbps) AAC had quite an advantage, even with LC-AAC. Since these tests, the Nero CLI AAC encoder has had a few updates, so it has potentially closed the gap; however I don't know how Vorbis is progressing, so it might be so damn close it doesn't even matter.
Mixed results, but you should be the judge ultimately.