The phrase Apple Music to MP3 320kbps is a quality shortcut. People want the densest MP3 a car stereo will take, not a 96 kbps YouTube rip.
What Apple actually streams
Apple Music’s everyday tier is AAC around 256 kbps. That is not MP3. Re-encoding to MP3 320 is a second lossy pass. It is still the right choice when the player only understands MP3.
When to pick M4A
If your phone, Mac or modern car reads AAC, M4A keeps you closer to the source. Convertol writes both, with the same title / artist / album / cover tags.
Convert on the home page — the quality menu is next to Download.