M4A is refused by some car stereos, older players, editors and upload forms. MP2 is the audio format broadcast and DVD-authoring tools expect. M4A is at home in music and voice memos on Apple devices; MP2 is the usual choice for broadcast radio and television audio. Both formats are lossy, so the content is compressed a second time; start from the best copy you have.
The defaults suit most files. Under Options you can choose the bitrate and trim the start and end.
This pair uses the compatibility engine, ffmpeg built for the browser, because MP2 cannot be written by the browser's built-in codecs. It is a one-off download of about 10 MB that the browser keeps, it is slower than the built-in codecs, and it takes files up to 1 GB. MP2 is written at 192 kbps unless you choose another bitrate.