It is much easier to find new releases and new artists on radio because they have a large dedicated new releases section which is updated weekly and ranked by popularity.
The radio option isn't something I use much but I find rdio works okay and spotify only works well for very popular music and bases itself completely off of artists so if you get some slow songs from rather not slow artists and start a radio you won't get any better an experience than rdio.
Rdio search also is much much faster on my phone than spotify which tends to hang forever.
I switch back and forth every few months to see if spotify has improved but it seems that rdio improves at a rate of 10x more than spotify because in the month that I try spotify rdio has already updated apps and web player to something that works better (see the ability to control rdio from any rdio app to the one that is playing).