I tried a whole bunch of streaming services when I was looking to jump from the paid version of Pandora (because Pandora streams at only 128kbps on all non-computer devices, including AVRs with built-in Pandora services, and that's for paid accounts).
For me, there is no contest: I find MOG
to be the best, by a good margin.
MOG streams virtually all content at 320kbps (very few are at 256kbps), while Spotify streams at mostly 160kbps. The difference is clearly audible on anything beyond computer speakers.
The MOG catalog seemed to me to be a bit better than Spotify's, too. For instance, when I compared them, MOG had a much larger Pink Floyd album collection
Since I've started using MOG, I have found myself almost never going back to my NAS-stored collection of losslessly ripped CDs -- the quality of the MOG stream is that good.
We pay $10 per month mostly so my wife can play it on the computer at work, but if you get mobile only account, it's $5 per month. At home, I stream MOG from my iPad to my HTPC (which is hooked up to my non-networked AVR) through AirPlay.
If your AVR is AirPlay capable, you can stream to it directly from any iOS device.