I've tried a few of the more popular streaming music services recently, including Spotify, Rdio, and Google Play Music All Access (what a mouthful). Here's a few thoughts on each.
Google Play Music has great sound quality, good music selection, a good UI, a reasonable recommendation system, and no iOS app.
Rdio has really poor sound quality, a frustrating to use interface, a really poor recommendation system, but pretty extensive library availability. Rdio is, in my opinion, the obvious loser in this group.
As much as Rdio is the obvious loser, Spotify is the clear winner here. Spotify has great sound quality (on extreme setting, anyway), a pretty good UI, a library that's slowly getting better. I can't stand some of Spotify's social aspects, and their recommendations are pretty bad.
If Google Play Music had an iOS app, it'd be the clear winner for me. I'd also pay more for it than Spotify. For some reason, it just feels more premium.