I usually use 160 as my low---the only real noticible distortion at that bitrate is cymbals, mostly. For stuff I really like to listen to, I rip straight to wav, though.
m4a is based off AAC. the compression methd for minidisc. it is a better compression method than mp3 (it throws away less data), but you will have compatibilties issues.
it is also a great compression rate for video (mp4).