Low bitrates: AAC vs Ogg, Need maximum efficiency

[Donadaif]

I need to fit as much music as humanly possible into a 8GB player that reads both formats, so I'm preparing to transcode my mp3 collection. Quality is not terribly important because I'll be listening to it on the go, but I still need it to be higher than mp3, because really low bitrate mp3s sound seriously awful.
Given a VBR setting that gives files with an average of roughly 96kbps, which of the two formats in the title gives the better results? How about at 80kbps?

I've googled a bit but I haven't been able to find a definite answer.

Thanks.