If they're both 320kbps they're both compressed the same amount. What they take out when compressing is different, but the actual file size is identical. I haven't looked at AAC vs MP3 in 5 years or so. Back then AAC was considered better as determined by picking out flaws in blind tests. However the encoders have changed so much since then.
Edit: Searching hydrogenaudio, listening tests show no appreciable difference between Apple AAC, Nero AAC, and Lame MP3 for 128kbps and up.
Edit2: One caveat. There will be some tracks that kill AAC and different ones that kill Lame.