Most of the tech writers I know who pay for a streaming music service use Rdio, not the more popular Spotify. Because we care about the technology and the experience nearly as much as the music catalog, if not more so, and well, Rdio's is better in just about every way.
To repeat a BuzzFeed talking point that my Sweet Slumber White Noise machine has been softly whispering into my ear every night as I go to sleep, Rdio gets social in a way that Spotify does not. I can actually see what my friends are listening to in a way that is useful, in that I can easily steal their good taste and pass it off as my own, spreading it to my other friends--no listmaking required.
Anyway, the canonical reason to choose Spotify over Rdio has been a question of the catalog: Spotify's is generally thought to be better. A deep study by Andy Baio, though, using data from the Whitburn Project shows that there's an incredible amount of overlap between the two catalogs--at least when if you're concerned with the top 100 songs, going back to 1955. Of the 5700 songs in the top 100, 88 percent were available on both services, with Rdio having 100 of the songs exclusive to its catalog and Spotify pulling in 81 exclusives.
It's a boon if your consideration is almost exclusively popular music and you're tired of Spotify's shitty experience. There are a couple of caveats. Namely, when it comes to deep and offbeat cuts, according to our music nerd friends, Spotify is still much better. Also! There's still weird holes when it comes to popular music, thanks to stands some bands are taking, like The Black Keys refusing to put its recent record El Camino on any streaming service because they weirdly see it as not paying for music.
Anyway, take a look at Andy's data. It's interesting. And if you have time, read this by Paul Miller, on the state of the streaming music industry. It's depressing, mostly.