I'm not entirely certain that I can tell the difference between MOG's 320 kbps and Spotify's ogg vorbis -q5 (160 kbps). I've never done a blind test, so I really can't say.
That being said, if only on principle, I believe that if I'm paying for a product, I better get what I'm paying for. MOG's library, at least for the US (at the moment), seems to be more thorough, at least for me. I'm finding album's on MOG that weren't available on Spotify (Someone Still Loves You Boris Yeltsin, Fleet Foxes's debut album, Radiohead's In Rainbows, Ra Ra Riot's The Rhumb Line). I haven't seen anything on Spotify that wasn't on MOG.
MOG's radio feature is pretty nice. Their feature request system seems really solid. They're going to start beta testing a desktop client soon, so that will be a substantial improvement on interface. You can trial MOG for 14 days for free. I would suggest giving it a trial. Even if you decide you don't care to pay $10 for premium on Spotify (if you don't care about bitrate or playing on a smartphone), you should still compare for the $5 price-point. I was enthralled with Spotify when it came out, but MOG is pretty solid.
I like how MOG has an "Editor's Picks" browsing category. The current web interface has browsing by genre and browsing for new releases. They're working on implementing the same browsing into their new Chrome player (which is really slick btw). If they release a downloadable desktop client, then MOG wins out for me completely. The delay between tracks is slightly worse in MOG, but I'm guessing a downloadable player would probably implement some kind of caching system to reduce such problems. MOG undeniably has a better system for browsing music. An equalizer has also been mentioned as a possible feature for the downloadable client. I suppose Spotify's client has an offline mode feature...but the advantage of that is limited if you have a solid, consistent internet connection. Both offer downloads for cellphones, and downloads for MOG are actually in 320 kbps (however, with the background noise likely in places where you would listen to music off of your phone, the improved bitrate may not really be appreciated).
Spotify does have a lot in the way of extra features though. Spotify Social connects with Facebook and makes it really easy to share stuff with your friends. More people use Spotify, so social features are inherently better on Spotify. Spotibot can generate playlists for Spotify based on recommendations that Last.FM makes. Websites like Pitchify.com track new releases and reviews and provide links to open the album in Spotify (but this isn't terribly useful for people in the US when a lot of the albums aren't available here). Spotify search plugins in Chrome allow you to highlight artist names in your browser, click a button, and automatically search Spotify's catalog (sweet feature to have while browsing releases on Wikipedia or Metacritic). ShareMyPlaylists.com has loads of playlists for Spotify.
So...yeah...try both and see which you like better.
UPDATE: I did a blind test on mp3ornot.com...apparently I can tell the difference between 320k and 128k...or at least there was only a 12.5% chance that I would've succeeded 3 times in a row. (3 times in a row on mp3ornot.com, then again on some other page, so 4 times in a row...~6% chance) I highly doubt that I would've been able to tell the difference on my Logitech X-240 speakers that I use on my desktop, but the difference is decently apparent on my Meelectronics AI-M6s. Of course, that's 128k CBR vs 320k CBR, not Ogg Vorbis -q5 vs q9, which are both VBR.
Edited by quadomatic - 7/22/11 at 12:27am