I doubt you'll use Pandora anymore. I kinda' gave up on it a few months back, it's library is fairly limited (I think something like 750k songs vs. 15+ million on Spotify) so after you listen to it for a while it starts recycling old songs, you never really get to hear any new or oddball stuff.
With Spotify, you have to manually choose the music or albums, it does't do it for you like Pandora, it's much more like iTunes, but just imagine having an iTunes library with every song ever. You can create playlists just like iTunes too so you can organize music how you want, which is cool. You can also share your playlists with people you're connected to on Facebook, which I also like. I've already discovered a dozen bands from shared playlists I would have otherwise never found.
It's pretty rad.
Have you checked out turntable.fm? It's pretty interesting too, worth a look, I just found it last week and have already stumbled on some new artists and new genres altogether.