Based on what I've read, Spotify has about 5 times the music (~5 million vs ~1 million)... I use Pandora when I want to simply listen to music on my laptop. Spotify is really useful for discovering new artists and songs... you can rank songs by a specific artist by popularity or sort them by album. It's a great tool when people post their top albums of the year and I've never heard of any of them. The Linux app sucks though.
