I have both paid versions.
I have been a user since basically launch in 2005 and I have a few stations that are that old and I have cultivated over the years. I think that if you use Pandora and give it a moderate amount of feedback, it will provide you with a great avenue to find new bands and things similar to what you are listening to already. The paid version lets you stream higher quality on a mobile device if that matters to you at all. There was a time when Pandora would only let you listen to 40 hours a month of music and that is why I got the paid version because I was blowing thru that in a week. There are also no commercials in the paid version. You are still limited to 6 skips an hour tho.
The "radio" feature SUCKS compared to Pandora in my experience. The listen on demand however, is awesome. I will usually call up an artist I find on Pandora and then listen to other songs from their catalog. I also enjoy the amount of music available because I have some weird semi local artists and even that music is there. The only advantage to paying for Spotify is the no commercials.
If you are looking for discovering music, then Pandora is the way to go IMO. If you want to just have the ablity to listen to what ever you want when you want then Spotify is good for that
Another thing to read is this article about Spotify and the music industry:
Taylor Swift: Why Taylor Swift Pulled Her Music From Spotify