mp3 is more universal for portable players, wma can be played on any windows pc, but so can mp3. Wma probably has the edge at very low bitrates but who wants low bitrates unless absolutely unavoidable? The quality difference is not great, wma just has a different frequency spectrum slice strategy and was an attempt by microsoft to usurp a popular free format with their own proprietary format. Both are outclassed by more recent encoding techniques such as aac, but with less universal playback options.