When it comes to choosing which option to go for, the most you can do is take each service for a spin, run a couple searches for stuff you like and see what comes up. You can get a general feeling of which one's a better fit and go with your gut, but wouldn't some hard data be nice?
The folks at Wired thought so, and decided to conduct an API-fueled study of Spotify and Rdio to see which service had more acclaimed music and which artists were exclusive to either service. The study took the API's from Spotify and Rdio and checked them against a dataset of 5,000 popular albums from user-generated music review site Rate Your Music.
The results show Rdio coming out on top by a number of measures, despite the fact that Spotify is known to have a bigger selection overall. Several respected artists were only availble on Rdio, including Bob Dylan, Pink Floyd, Queen and Thelonius Monk. Both services have a ton of exclusive albums, but only Rdio can boast Pink Floyd's Dark Side of the Moon or London Calling by The Clash.
Spotify (at 4.8%) had slightly fewer exclusive albums than Rdio, on which 6.8% of the albums were available exclusively. Nine of the 100 most popular albums were only on Rdio, while only one of them was exclusive to Spotify.
To be fair, Rate Your Music is probably not the most authoritative source of what's popular. A more complex analysis might mash together datasets from Billboard, Last.fm and, if possible, Amazon user reviews to come up with a more comprehensive list of popular albums.
Not included in the study were services like Grooveshark and MOG, the latter of which does not make an API available to developers.