Depends on the application... but maybe http://dev.gentoo.org/~redhatter/misc/mab135-project.pdf will help your decision process.

I actually did a comparison between MP3, Vorbis and AAC as a university project. Using analytical means, it was determined that for an AAC file at 128Kbps, Vorbis was roughly equivalent at 160Kbps, and MP3 was equivalent at 224Kbps.