I have never run out of music with MOG, but I have a paid account.
Spotify is the main competition to MOG. However, it was slower to go to 320kbps, slower to the USA, slower with iDevice 320k streaming, and slower to come out with an iPad app. While Spotify has caught up, I prefer staying with a leader instead of a follower. Without MOG, I doubt Spotify would have made the strides to catch up.
I do have a free Spotify account, but I can't remember the last time I tried it.
The other option is a Google Music Play. I tried it, but I didn't like the fact that you have to access it from a web browser on both the computer and iOS. I prefer an app that can be installed and accessed outside a browser.