Is Spotify a bigger threat to Google's ambitions for music on Android or Apple's upcoming cloud-based music service?I would say so. One big reason for Spotify's huge user growth success in Europe is because their lightweight, cross platform client software. If you compare that to Apple iTunes its definitely will be difficult for Apple to compete with that, unless they would release a iTunes "light", which most probably is not likely to happen. Another important factor is that Google Music will have hard time competing on the market, considering that both Apple and Spotify already have established several years of relationship with both artists and record labels all over the world. Content is king and one huge advantage that Spotify has against any competitor is that several of the worlds largest record companies actually owns a stake in Spotify Ltd.
You have forgotten about Amazon's Cloud Service. They recently opened up to allow unlimited songs as long as you have one of their higher services. I've tried Spotify and the only thing I could do is play the songs on my personal computer. If I wanted to do anything else, I would have to pay. You currently don't have to pay with Google Music. I had a $5 gift certificate for Amazon and so, didn't have to pay for it either. What does Spotify do that iTunes or these other services don't already do? I scrobble my tracks to Last.Fm via a plugin for iTunes. I don't need Spotify for that. For me, Spotify is late to the game and doesn't offer anything new. Nothing game-changing despite the long wait here in the U.S.