I followed MP3 and Vorbis development in its early days. From my perspective as an end user myself, I thought that MP3 encoders (LAME, especially) improved by leaps and bounds in the transparency front, while Vorbis remained largely as a low-bitrate (128kbps~) alternative.

Plus, I could play MP3 on virtually any device. Vorbis support depended on iRiver releasing new firmware, or hacking an iPod to support Rockbox.

The Hydrogenaudio forums [1] are a great place to see how the formats progressed, if you're willing to dig through the archives.

[1] http://www.hydrogenaudio.org/forums/index.php?act=idx