Not much to add here except to say that I'm in the middle of sorting this out tonight as well and would love to hear what others thought. I'm listening to Spotify tonight, but have also read some comparisons tonight that give the sound quality nod to Mog because it streams 320 at $4.95 while Spotify only does 320 at the "premium" $9.95 level. I've tried stumping both with weird artists and they both are surprisingly deep, although I did see a review that gave the depth-of-library nod to Mog mostly because Mog had the entire Pink Floyd and Bob Dylan catalogues and Spotify didn't. The reviewer claimed that Spotify had a hole in it's classic rock catalogue.
I'm a little new to the quality pursuit - I just bought a Schiit Asgard, a DAC, and some Sennheiser HD598's so I'd like to know whether the difference between 160 and 320 kbps is as big as it would seem.