For various conversions, I find
XRECODE (FREE!!!) very handy. It comes with the LAME encoder, so you don't have to download & install it separately. (The current version, XRECODE II, is $15 USD.)
Since both AAC & MP3 are lossy formats, I recommend using a high MP3 bitrate* to minimize any additional quality loss.
* With LAME's variable bitrate settings, you don't actually select the bitrate, you choose a quality setting (V0 is best), and the encoder chooses the appropriate moment-to-moment bitrate. (It's generally best to use variable bitrate & joint stereo.)