I prefer m4a. I can handle mp3. And I can convert one to the other. Am sure most people listening to audiobooks have use both or would easily learn how to use them. (even though old mp3 players don't reproduce m4a, so that's a thing)
So I vote for whatever is technologically easier and cheaper for Paul and team. Cause it's better to delegate little efforts than to centralize a big investment. :)
