I am really excited about the new iTunes Radio that is coming out this fall, and I think that it will definitely be giving both Pandora and Spotify a run for the money. It looks to me like the iRadio will be a much better choice than either Pandora or Spotify, which is already very expensive to get their premium service.
I just have the free edition of both programs, but since I am usually on my iPad, and Spotify really doesn't work there, unless you have premium, I only listen to Pandora.
I have iTunes Match, so that will make iRadio Premium free, and no ads, and I can use it on all of my devices, so obviously, it is going to be the best choice for me.