I've also been a MOG user for a while now. The Android app and Roku channel are so-so, but their "Chrome" app is absolutely stellar, and is the one I use most often. I was eager to try Spotify, as a Norwegian I'd heard and read a lot about it, and the hype has been running overdrive the last several weeks.
I must say I am underwhelmed with Spotify. Their PC app, though very nimble and FAST, is terrible when it comes to organizing music. The Android app good and reliably streamed music for hours while at work. As for the library, at first I was thrilled. I found a lot of Norwegian music that simply do not exist on other services, and I've been listening to that a lot over the last several days. However, then I started to look up many of the favorites I have in MOG that I regularly listen too, the gaping holes in the Spotify library became apparent.
First one I looked up was Alison Krauss and Union Station, looking for the latest release, Paper Airplane. It's not on Spotify. So I googled it, and found a Spotify link to the album. When I clicked on it, it opened Spotify with the album and the message "This album is not available in United States".. WTF? It's been on MOG since it was released.
Further searching, Paul McCartney, there are a few albums on Spotify, but nothing like MOG. Spotify does not have McCartney I and II, Band on the Run (all 3 recently remastered (in a good way) and available on MOG in remastered form).
On to European power metal... no Visions of Atlantis. Next, Within Temptation, Spotify only has a couple of albums (missing the last two studio albums). MOG has them all.
At this point there's no way I will give up MOG for Spotify.