Regarding reach, inventory and ecosystem - I think Spotify should be very worried by Apple. The iTunes library is one of the biggest and it seems that most of it is available for streaming. Apple Music will be coming preinstalled on all iOS devices (with three months of the service for free) and, to top it all off, it'll be available on Android too. I'd be very concerned if I were Spotify.
What's more, iOS users tend to be some of the more valuable customers, and I wouldn't be surprised to see a higher portion of Spotify Premium subscribers on iOS than other platforms. I suspect that Spotify Premium memberships will take a big hit after it launches. I know I'll be leaving for it.
To me, Spotify is a perfect example of the power of a good UI (or rather, how damaging a poor UI can be). I have subscribed to Spotify since pretty much the beginning - for years and years - and yet I am completely disloyal to them. I can't think of any other company, besides utility companies and such, where I haven't become somewhat fond of them having been a customer for so long. I can't stand Spotify. Whilst the service is great, the UI is awful and the apps are slow, don't feel native and are annoying to use. That's why, having only seen screenshots of Apple Music, I know I'll be switching to it from day one. A good user interface is king.
(In case anyone brings it up, I've tried many competitors to Spotify, including Rdio, and haven't been able to find anything better. Rdio looked promising but their Mac app is actually even worse.)