Once upon a time, Sirius XM (SIRI -3.39%) was the elder statesman of digital radio services while Spotify (SPOT 2.86%) played the hungry upstart. As time went by, their roles changed. Now, Spotify has a $50.6 billion market cap and$13.3 billion in annual sales. In the opposite corner, Sirius XM weighs in at $17 billion and$9 billion on the same metrics. Spotify's fortunes have soared while Sirius XM's starlight faded out. But past performance is no guarantee of future results, and the picture in the rear- ...