AFAIK, MP3Pro is only marginally better-supported than WMA (the "backward compatibility" is utter junk; put it out of your mind now). Go with Ogg Vorbis or MP3 if you care at all about people actually being able to play it.
how about shorten (*.shn)? Completely lossless compression, but only about 1:2 compression ratio. Its basically the standard format for live show trading because any other compression would quickly render many live recordings unlistenable. I like it for what it does, but for casual downloading and listening I will stick with regular mp3s.