For people who like to listen to a wide variety of music and like to discover new music, I don't think you can beat what Spotify offers. If you have a more tightly contained bucket of music and don't want to expand in too much, it probably makes more sense to own your collection.
I've discovered a ton of stuff and enjoyed things one-off like live CD's that I would have never have purchased outright through iTunes.
I ultimately chose Spotify over Rhapsody because of playlists. I think when I first started, I actually prefered Rhapsody's mobile UI but Spotify's has gotten better since. I remember using Rhapsody's web/desktop service in the mid 2000's, but it wasn't an iPod replacement then because it's offline capabilities were limited and limited to devices that I didn't like. Once mobile became realistic to stream music, the services started making a lot more sense to me.