In regards to subjective sound quality of Spotify vs. Mog, I was listening to Homecoming Heroes by the Head and the Heart on both Mog and Spotify and noticed that Mog had a significantly fuller, detailed sound. I thought this must be due to my excitement about discovering Mog, so I put it to a subjective blind test. I played Another Story by the Head and the Heart and Dirty Paws by Of Monsters and Men for my girlfriend on both Mog and Spotify. I just told her I wanted her opinion, without mentioning anything about Mog or Spotify. Both times she picked out the Mog playback as the better sounding recording within 20 seconds.
For what it's worth..
MOG does an excellent job encoding their 320 kbps MP3s. Spotify quality is more uneven and I read that some of their high bitrate catalog is upsampled from the low bitrate files.