Here is my review of all these services, it has an Australian twist with the recent launch of Mog in Australia but everyone should find it useful.
After using Mog this morning, and being a heavy user of both Rdio and Spotify here is my take on the new service.
Music Selection
It's hard to tell at this stage but the selection seems very large, comparable to Spotify. Rdio has the weakest selection of the three. Telstra have made a priority to ensure Australian content is there and I haven't noticed any of my most listened to music as missing. Rdio is the biggest disappointment here with many artists unavailable in Australia, I assume there are some licensing issues that both Spotify and Mog have overcome that Rdio has not secured.
If I was basing my decision on music selection alone, it would be a toss up between Spotify and Mog. And I'd probably give it to Mog in a pinch with more Australian artists.
User Experience
Firstly I applaud Mog for having both a web app and also mobile apps for both iPhone and iPad, and Android too I believe. The interface on both mobile and the web seems a bit basic and sparse, and not in a beautiful minimalist way. Not to say it's ugly or unintuitive, but after using Rdio for so long I have been spoiled. Rdio has by far the nicest presentation of the three services. Using Rdio truly feels like the future of music discovery and consumption, while Mog offers a more basic interface.
Comparing Mog to Spotify, I think it's on par actually, even though Spotify has far more buzz and users. Rdio outshines Spotify, particularly with the iPhone app. Both Mog and Spotify have recently updated their iPad apps and the iPad is the platform where all three services are the most similar in terms of visual appeal.
Spotify's apps are strangely lackluster for such a huge service. Their new iPad app is nice, borrowing design elements from Twitter for iPad, but it doesn't blow me away. Their iPhone app feels like it was designed in 2007 and not updated since it's a bit old and crusty looking. It get's the job done but it won't excite you.
In terms of user interface and experience, Rdio would be the runaway winner.
Music Discovery
Rdio has been the long time king of music discovery among these streaming services. It's Heavy Rotation view which highlights the most listened to music among the people you're following is a great way to find new artists and albums. Additionally, Rdio offers Top Charts, Recent Activity among your friends and Playlists that others create that you can subscribe to.
Spotify also offers Top Charts and Friend Activity, but it doesn't have a similar Heavy Rotation like Rdio. What it does have though, is Radio. Based on an algorithm using your friends music listening habits coupled with data taken from worldwide users, radio is able to stream music based on a certain artist. And it keeps on going and going like your own personal DJ. I've found this to be a great feature though I still prefer Rdio's heavy rotation for finding new music.
Mog has a radio function that works very similarly to Spotify. It also offers the usual top charts and friend activity, and additionally it offers Editors Picks to help you discover great new music among the avalanche of music available.
While Rdio has been generally the king of music discovery, both Spotify's and Mog's new radio features certainly help them get a lot closer. In fact, when I'm using Rdio I find I really miss this no-brainer of a feature.
It's hard to find a winner in this category, but I'm going to still stick with Rdio.
Other Details
There are some other unique features each service has that are very much worth noting. Spotify has a strong app selection, both on desktop and third party mobile apps. These apps include Last.Fm, Triple J and a radio app called Spot On on iPhone. They really help to expand the functionality of Spotify, and some of these apps and services are more beautiful than Spotify's own apps. It also really helps beef up discovery of music on Spotify. Apps on Spotify are a big deal and big leg up on both Rdio and Mog.
Mog's launch brings unmetered downloads to Telstra customers, and if you're a Bigpond broadband or mobile user that's hard to ignore. I have a 3gb cap on mobile NextG data, and these streaming services can really eat into that. Not having to worry about eating my data while using Mog is almost a killer feature for me. Your results may vary, if you're not a Telstra customer this has no benefit for you, and if you are on Telstra maybe the extra data usage for streaming music is not a big deal especially if you're on a large cap.
Conslusion
I'm still undecided to be honest. Each service has it's own pros and cons. Rdio, with it's beautiful interface and apps and fantastic music discovery. Mog with it's unmetered data usage and wide song selection. Spotify with it's wide usage, app selection and strength in the market.
You have to look at all these aspects and decide which service is right for you.