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I use the free streaming radio on both Pandora and Spotify. I find Pandora tends to give me more new artists. Spotify tends to play artists I already know, though often tracks I don't have in my personal collection. I like that Spotify links to my Last.fm account.

Which one I use just depends if I'm in the mood for artist discovery or not.
 
I like pandora, i have created several stations and then just shuffel play all of them. So in an hour of listening i hear top forty, classic rock, outlaw contry, pop standars from the 40's and 50's and irish folk songs.
 
Rdio is better than Spotify. As for Pandora, I'll never understand why you would want someone or something else picking your music for you.
 
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