According to studies, at 256-320kbps there is no audible difference between the two formats.
Apparently AAC is noticeably better as the bitrate lowers from there, but mp3 has a smaller file size at all levels of compression.
Since you can't convert from one lossy format to another without even more loss of fidelity, and as there is not enough advantage to AAC to warrant re-ripping your library from a lossless format, I think I will be sticking with mp3 for quite some time.
The MP3 is dead! Long live the MP3!