While I've got Amazon Prime for streaming music I don't like that aspect of it too much - I like to be able to "thumb through" my album collection until I see something I like. Having the entire world of music at my fingertips, I might be stuck what to pick Amazon lets you add Prime (free) stuff to your collection, no idea about Spotify?
Also, Amazon streaming music is flaky for me which makes me dubious. On both iOS and Android devices it gets the odd stutter and sometimes playback will just hang until I skip to the next track. Which is crazy with Fibre... bad developers I reckon.