i would have said wma has better quality.
mp3 works by removing sounds that can not be heard. eg frequencies outwith human hearing range (20-20khz) and removing quiet noises from a piece that may be drowned out by other things. after all if you cant hear it whats the point in having it stored?
im not sure entirely how wma stores it but it is different from this.
so i would have said wma because: although mp3 removes sounds you cant hear, removing sounds still has a small effect on the piece of music. so file size irrelevant: wma.
if file size is a factor, mp3 all the way