But there is a BIG difference here if we're talking about the free services of both. Spotify is always free, and rdio is only free for 6 months. Yes it's probably a good thing to pay for a service that you like (I do), but some people don't, and that's going to be where they prefer Spotify.
http://support.spotify.com/us/learn-more/faq/#!/article/How-...
http://help.rdio.com/customer/portal/articles/187730-rdio-fo...