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I too would like to see such a comparison but # of songs isn't the important metric for me, it's # of songs I want to listen to. That's why I hate that metric, really just tell me which artists you have (or don't have, though this would never happen) that other services do not...

To me the killer feature combination of Play Music is the unlimited streaming + the music locker.

If something's not available for streaming, or in the store, you can upload it yourself.

They all have almost same (number of) songs.