You forgot that grooveshark has horribly and inconsistent audio quality and that it is borderline illegal.
EDIT: Now also sent to CephaSquiddy, Colten, kakashi11 and Dissolve.
20 invites left.
How does Grooveshark differ from Spotify legality-wise? Both are pretty much the same thing.
Audio quality is perfectly fine for me, I'd say Spotify had pretty much the same quality. Bitrates probably differ, but you can't tell the difference when listening. You just probably found a track with bad quality, Spotify has them too.