M4A is refused by some car stereos, older players, editors and upload forms. AIFF is uncompressed and native to Mac audio software. M4A is at home in music and voice memos on Apple devices; AIFF is the usual choice for audio production on the Mac. AIFF cannot bring back detail that M4A compression already removed; it simply stores the result without further loss, in a larger file.
The defaults suit most files. Under Options you can trim the start and end.
This pair uses the compatibility engine, ffmpeg built for the browser, because AIFF 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.