this is a proprietary format developed by Microsoft. Users have noticed low error resilience: sometimes insignificant damage to the file can cause the file not to play. Besides this, the closed format hampers its proliferation outside of Windows platforms.
this format is free, which apparently is one of the reasons for its popularity. The used codecs supports variable bit rates (as a default), and .ogg files usually play back easily on all platforms. All things being equal, the sound quality exceeds that of mp3 somewhat.