As far as catalogues go, both seem to be pretty equal (I'm basing this opinion on the European version of Spotify)- you won't find the Beatles, AC/DC and Ozzy-era Black Sabbath on either, but MOG has recently been beefing up its classical, opera and jazz catalogues to the point that what was once a huge gap for me is now a trivial issue- there are at least seven major recordings of complete Ring cycles (Bohm, Levine, Boulez, etc.); it looks like the entire Maria Callas catalogue from EMI classics is now available, along with most operas on RCA and EMI; the major works of Glass, Adams and Penderecki are there as well. You get the idea.
There are irksome quirks, no doubt due to these services being oriented toward the pop consumer. If you search on "Verdi" you'll bring up those crappy D'Oro recordings, but not see the Aida with Price or the Don Carlo with Domingo, both of which show up on a "Domingo" search. You can find Ligeti's Le Grand Macabre if you search for Esa-Pekka Salonen but it doesn't show up in a search by the composer. In other words, searching by specific performers or operas yields much better results than searching by composer. If anyone from MOG is reading this, I can be hired to help you out with this problem.
Updated 07/18/12: Additionally, there aren't ways to combine your existing library into MOG and you can't listen to anything offline without an IPhone, though a reader says you can do both on Spotify. Still, for most users these aren't big deals. However, since I just got my Spotify invitation today I'll check it out and compare the American version to MOG. Both services let you buy the music- Spotify from ITunes, MOG from Amazon (which is usually cheaper)- and you can create playlists (a shuffle for these is a missing feature in the beta player but it's in the original).
MOG has three other things going for it that will keep me in the fold: first, it has great customer service that has responded to any question I've asked with a real answer within 24 hours; second, the company is based in Berkeley and I'm going to root for the local team whenever possible. There also some social media capabilities like following other users, which I'm less involved with and may be stronger on Spotify (but you can follow this blog on MOG), which seems to have better integration with Facebook at the moment. Both are superior to Pandora, Last and the others, and being able to play an entire album in one click or make a playlist and then let it roll is a far better experience than calling up one track at a time on something like MySpace (I'm talking to you, FF). Also important, but dependent on so many factors dependent on the listener's environment and equipment, MOG has better sound quality than Spotify. The other day I was listening to the first Code Blue album and the force and quality was fantastic. I've also been listening to a lot of the remastered Stones albums lately and MOG has a depth of sound which Spotify doesn't equal.
Unless you're an audiophile (which means you aren't even actually considering listening to any recording originating from a stream in the first place) or feel the need to support artists (and major labels) by giving them your cash on principle (which means you're still living in the 70's), this is now the way to listen to music. So click that link to the right and hear what you've been missing. But be prepared- once you start searching for music you haven't heard in ages (Rubber City Rebels, anyone?) and finding all of it, as well as giving all that new stuff you've been reading about a try (Bon Iver just made me depressed), you'll suddenly wonder where all those hours went.