I'm hard pressed to think of a reason to stay with Pandora. For free on Spotify, you get Pandora-Like radio AND you can put together your own playlists of songs that will "shuffle" play. So you can pick exactly the music you want, just not the order it plays in.

For a fee, Spotify will allow you access and control over their whole library.

Despite this, I still use Pandora most of the time.