ffmpeg can decode m4a files quite easily. But, I'm sure about m4b. It depends on the version of ffmpeg and what decoders/encoders was compiled into the binary.
You can check the decoders/encoders used by ffmpeg by typing this at the terminal
ffmpeg -formats | egrep -i 'm4a|m4b'
To convert m4a files, type this
ffmpeg -i input_file.m4a -acodec libmp3lame output_file.mp3
Ffmpeg will use the audio values of the input file for the mp3 file, If you want to change the audio quality of the mp3 file, you can use these switches
-ab = audio bitrate # example -ab 192k
-ar = audio frequency # example -ar 48000
-ac = audio channels # example -ac 2
On some distros, ffmpeg is depreciated and avconv is the replacement for it. However, avconv is compatible with ffmpeg switches as well.