Here's my pros and cons list:
Spotify is better if you want to send an album to a person -- their inbox feature is really cool. You can see a facebook friend, send them a song in their "inbox."
With Rdio you'd have to send a link a more traditional way -- it's not built in.
I think Rdio's interface is far, far, far better. It's more intuitive, and it let's you find and play full albums better. Spotify is better at just finding a song to play.
Rdio does not give you access to your local library in their web or desktop apps -- Spotify does. I find myself switching between rdio and iTunes during the day to play something I have on my hard drive and something I don't. Not that big a deal, but hopefully something that someone changes someday.
Rdio doesn't have any ads.
/my 2 cents.