If you save as .MUS, then only people with Finale-family products can open it, but they will be able to edit it.
If you save as .MID, anyone can open it, but you lose a lot of (non-musical) data (lyrics/etc.), so if someone wanted to edit it, they might be annoyed.
If you buy a higher-class Finale-family program (for example, Printmusic), you could save as WAV or MP3.
If you save as .MUS, then
If you save as .MUS, then only people with Finale-family products can open it, but they will be able to edit it.
If you save as .MID, anyone can open it, but you lose a lot of (non-musical) data (lyrics/etc.), so if someone wanted to edit it, they might be annoyed.
If you buy a higher-class Finale-family program (for example, Printmusic), you could save as WAV or MP3.