Music discovery/radio services: What about Songza?

I've been reading a lot recently about Rdio, Spotify, Xbox Music, and others - but I've heard (at least in my browsing and reading experience on the web) very little about Songza recently (or ever, actually). I actually stumbled across the app when I saw it on a relative's iPad, after which I did my quick 5 minute googling to see what it's about and whether or not it's worth it.

And now I turn to the Verge community for your opinion. Why Rdio and not Songza? Spotify is not offered in my country, so that's not really an option for me. The only reason I can think of (based on my 10 minutes using Songza just now) is that you can't specifically select which album/artist you want to be played on Songza, and are instead given set playlists with random songs you may or may not like. Otherwise, I see no other reason to choose Rdio over Songza - especially since Songza is free, and ad-free.

In short, what can you guys tell me about Rdio that makes it worth spending $10 a month on, compared to Songza's free service? I've been looking to join Rdio for a while, but after seeing Songza... I've been forced to double-take on it. Any comments or further insights on the two services would be much appreciated!