The other component that makes Spotify intriguing is the social media aspect of it. You can see what all your FB friends are listening too and even share playlists.
Overall, I just think it's ability to stream and store (you can keep over 3,000 files local for when you're in airplane mode or have degraded service) high quality files, its breadth of catalog, its price point, and it's FB integration, makes it the best option. Plus, unlike Sirius, you can cancel at any time.