A search for these.
Best audio MP3 or AC3
Best audio AC3 or MP3,
brings up converters, nowadays most music is MP3, but converting a movie has the option on choosing the
audio, either AC3 or MP3.
thanks.

AC3 is Dolby Digital.

Assuming you are ripping from a DVD leave it as AC3 as no transcoding will take place.  Therefore no loss in quality.

What is the source? What is the target play back device?

Neither is best, it depends what your source is and what you want to do with it.