All information I found online indicates that AAC is superior to MP3 in every aspect, having better sound quality at lower bitrates/file sizes etc.
So is there any reason to ever use MP3 instead of AAC?
There is always the argument that MP3 is more supported, but I personally think that that argument doesn't hold anymore today. All "modern" multi media devices support hundreds of different video, audio and image codecs, including AAC.