RDIO and MOG are very similar in what they offer. They both have a $5 PC plan, a $10 "mobile + living-room + offline" plan, PC/Mac apps, a radio mode, they have mobile apps, a Sonos app, and a Roku app. RDIO wins in discovery, usability/UI and app stability over MOG (MOG's Android app continues to be unstable for me since my last article here on OSNews, despite MOG contacting me and telling me that they're fixing it). However, MOG has a substantially bigger catalog, and higher audio quality. RDIO refuses to disclose their bitrate, but some users online claim that it's no better than 128 kbps (we don't know for sure), while MOG's ranges from 128kbps to 320 kbps depending on the quality of the connection.
Spotify on the other hand has the app stability, and UI expected, but with music discovery and social networking being only so-so. Their quality ranges from 96 kbps to 320kbps, depending on the plan or wireless technology used. They feature the same plans as MOG and RDIO, but with the addition of a free plan that allows for 10-hour a month of free streaming, with some ads (that's enough for 12 or 13 albums of free streaming a month). Spotify also has a Sonos app, but not a Roku app. Spotify also works in more countries than just US, so if you're traveling in those countries you won't have to always use "offline" synced mode, but live as well. Spotify can also integrate your own local music into their player, not just stream. It also features PC/Mac apps, a radio mode for its paid plans, and it has broader mobile support for more mobile OSes.
So if you try to weigh all three services, you'd likely say they're about the same, with Spotify having an advantage, with possibly RDIO getting a 2nd place. However, it's not so. Spotify/MOG leap ahead to a huge advantage over RDIO in regards to their catalog size. At the end of the day, if your favorite artist is not part of the service, having a nice social networking integration won't do jack to keep you satisfied. That's what's killing RDIO right now: their limited music catalog. They have about 8-9 million songs, while MOG has about 11-12 million, with Spotify having about 13 million (15 million according to some news outlets).
Through the last few months that I have been subscribing to RDIO, I've routinely been unable to find many indie bands. I'm an indie music enthusiast, so every Tuesday I'd browse the iTunes Alternative genre listings, to see the new releases. I'd preview stuff there, and then, for whatever I liked, I'd go back to RDIO and add them in my "Collection". Well, bad luck for me, I constantly found ~1/3 of the artists I was looking for to not be available (either not listed at all, or being "greyed out" in the RDIO UI). RDIO would direct me to their catalog requests page, and there I would fill up their form. None of my requests have made it to RDIO yet, so at some point, I gave up filling forms.
However, I did keep a list of 20 artists I filled on the RDIO form early on, so I used these artists to check MOG and Spotify. I found out that from my small sample of 20 artists that were missing from RDIO, both MOG and Spotify covered 15 of them, and missing only 5 of them. The same 5 artists. The universally missing artists are: Secret Cities, Teen Daze, Pure X, Holy Other, and MemoryHouse (most of them self-released, or on small labels, but they all exist on iTunes, and 3 of them exist on Amazon MP3). Spotify fared better than MOG in the album selection of artists that exist in both services, e.g. Spotify had more albums/songs by AG Silver than MOG had. RDIO does not carry AG Silver at all.
I'm personally considering unsubscribing from RDIO because of their weak catalog, and their reluctance to fix their Roku app that has a reproducible crashing bug (they were notified months ago to fix it, nothing was done yet). I'm torn between MOG and Spotify, because while I prefer Spotify overall, they don't have a Roku app (I have wired my Roku to our Yamaha amplifier, so I can stream to our huge speakers in our living room). See, to me any service is incomplete without some living room support, that's where music can be most appreciated (failing a live performance at a club). If Spotify were to offer a Roku app soon (and possibly the ability to control the Roku app from the Android app -- with my TV OFF), there won't be any question, Spotify is where my future would lie.
As for RDIO, if they don't have the funds to sign more labels/artists, I think they should enter in a strategic partnership with Bandcamp (down the street from RDIO's offices), to at least get the kind of artists that are usually not signed in any label, and no other service offers (not even iTunes).
Finally, I'd like to say that having used every kind of music service out there, from local playback (iPod), to radio (Slacker, Pandora, Last.fm), to unlimited streaming (MOG, RDIO, Spotify), to cloud-services (AMZ, Google), I much prefer the unlimited streaming services. I have 150 GB of legal music that I can't fit on any cellphone, radios don't play exactly what I want, while cloud services not only can't fit my music for a low price, but the upload times are so high that it makes them non-worthy of my time. With unlimited services I can get 90% of what I want to listen, for a very small fee, and I can fill up the rest 10% with local playback (I usually buy from Amazon MP3's low prices, but most of the indie stuff I listen to are legally free for promotional purposes on various well-known music blogs, or on Bandcamp).