I believe that Apple abandoned their proprietory protection and just distribute music files as .mp3 but with their proprietory three letter suffix instead added to the file name. Don't have a PC to hand, but I believe .mp3 is more compact than .wma format files.
I use .mp3 format by default because it is recognised and simply works across multiple devices, even if it doesn't give the highest fidelity music reproduction. It is good enough for the vast majority of listeners.