What is M4A?
M4A is AAC audio in an MP4 container - what iTunes sold, what Apple Music and most podcasts ship, and what a voice memo becomes. At the same bitrate it sounds better than MP3, and it is read by rather less hardware.
What is MP3?
MP3 is the format that made digital music portable, and thirty years on it is still the safest bet that a file will simply play - on any device, in any car, in any decade-old piece of hardware. It is lossy, and its patents expired in 2017.