Shoutbox

m4a is only a filename extension invented by Apple for MP4 files. m4a is not a filetype or codec

I take it you mean AAC audio (inside of an MP4 container) versus Windows Media Audio (WMA).

AAC compresses sound better than WMA, and is also more compatible.

MP3 compares to AAC fairly well at higher bitrates, and is more widely supported than AAC or WMA. It also often consumes less power, so sueful for portable players. It's worth considering if you can use it and won't need tiny bitrates.