Spotify or Pandora
 

I liked Spotify Mobile, but I felt like it required me to know what I wanted to listen to (and pick songs) or make playlists in advance (I'm kind of lazy). Pandora is better for me since I can pick a genre/artist/song and it will generate me a bunch of similar songs for as long as I can listen. The ads are manageable but for 1/10th the price ($0.99), I could remove the ads entirely for the month.

Spotify just introduced their Radio functionality on mobile for free, but I have never been impressed with it compared to Pandora for matching. I will be trying it out more now that it is on mobile, however.

What I am really waiting for is the next iPhone with 4G LTE. I have unlimited data on Verizon and will get to keep that as long as I buy the phone unlocked. iTunes Match (iCloud) is great but it's spotty over 3G. With LTE, I'm sure I could stream my entire library and use Genius Mixes/Playlists no problem.

In my opinion, Pandora is far better for music discovery. If I'm my electronic moods, I'll pick something like Daft Punk, Nero, Feed Me, Deadmau5, SebastiAn, or the like. Lots of times I encounter new songs and artists and I go home and check them out on iTunes. I'm trying to build up my library as much as possible in anticipation for what I mentioned above.

Now Spotify, if I have a song stuck in my head, I can find it on there and play it 9 times out of 10. That can be handy, but I typically can distract myself with something else off Pandora instead.

I use spotify when I know what I want to listen to and pandora when I just feel like listening to anything. The quality of spotify is much better than pandora over 3G. I also find sharemyplaylist is great for discovering new music on spotify.