I saw a chart a few months ago and I wish I could find it again. It's a break down of all the different music app/sites. It had the number of songs, bit rate, price, etc.
After using Slacker for months, I've been using MOG a lot of late. The app is a little more intuitive but the "radio" feature doesn't appear to be as refined.
David
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Also, for some reason MOG has never charged me. Not sure what's up with that.
I use MOG a lot and love it but also use free Spotify. MOG is really an excellent product and their iPhone app is awesome. But as you mentioned, the "radio" is just not up to speed when compared to Spotify. I am lazy sometimes and don't feel like picking out songs and it is easy to just run the Spotify genre radio and not worry about it. Sometimes I am tempted to switch to Spotify just for the radio and improved bitrate over their free product.