I'd really like to see that comparison as well. And wouldn't Pandora and Spotify, like radio, be a channel to expose people to new music that they then might buy? Personally, Pandora has worked this way for me multiple times--they'll play a track that I haven't heard before and like, and I'll go to Amazon and buy the track or sometimes the whole album.

In the days when a few radio stations were the only free mass distribution channel, this exposure was so valuable that bands or their labels would actually pay the radio stations substantial sums to have their songs played. This was known as "payola," and was viewed as a crime, for reasons that are still a bit unclear to me.