Slacker Radio, an Internet radio service similar to streaming music giant Pandora, has just introduced a new tier to its subscription service: a $9.99 per month Premium version which offers music on-demand. Previously, as the name implies, Slacker Radio was more focused on a radio-like experience, where you listen to a station built around your favorite artist, just like in Pandora. For example, type in "Lady Gaga" and you'll be presented with her hottest tracks, as well as those from "related" artists.

To ditch the advertisements and skip songs you don't like, a $3.99/month subscription (Slacker Radio Plus) was made available. And today, Slacker has launched another option: a $9.99/month Premium Radio subscription for playing the songs, albums or artists you want to hear on demand. This is similar to a number of other services out there today, including two of our favorites, MOG and Rdio.

Which one is right for you?

Yes, We Know: There Are Other Streaming Music Services

MOG and Rdio are only two of the cloud-based streaming music subscription services available today, so this isn't meant to be a comprehensive review. Others, like Napster, Grooveshark, Spotify, Rhapsody, Microsoft's Zune and others are also available, with similar pricing.

But MOG, Rdio and Slacker are, let's be honest, some of the newer, and cooler* services** these days (for U.S. users***), each offering unique features, from social sharing of playlists to free cloud storage for your own tracks and more.

*Cool, as you know is an entirely subjective word. Did we mention this was a blog?

**We think Grooveshark is pretty cool too, but is still facing questions about its legality. The company maintains there's nothing illegal about its service, but in truth, it's operating in a gray area of the law and has since been pulled from iTunes because of that. Spotify may be the coolest of the bunch, but is not available commercially in the U.S. market. Napster and Rhapsody are nice too, but are old brands doing new things, as opposed to new startups doing new things. That doesn't make their services unworthy of consideration, but they are not being reviewed in this article.

***Yes, this review is U.S.-centric, sorry overseas readers!

What is Slacker Premium Offering?

With Slacker, the new Premium service offers over 8 million tracks from the four major labels and thousands of indies. You can search, play and replay specific songs or entire albums. And you can create playlists both online and on your mobile. Currently, Slacker has apps for iPhone, iPod Touch, iPad, Android and BlackBerry phones. Offline playback is supported on these devices, too.

Users can also access the 150+ pre-programmed Slacker Radio stations based on musical genre, and can view and play the top 50 most popular songs for that genre's station.

While listening to music, users can favorite tracks for easy retrieval later. And like the Plus offering, this service is available ad-free, and with unlimited song-skipping. It even offers the ability to view song lyrics.

Slacker notes on the subscriptions page of its website that with 8 million tracks, Slacker has 6 times more songs available than its top competitor. That's a fair comparison, given that Pandora's catalog only includes 800,000 unique songs. While not all of Slacker's 8 million tracks are available on demand - that's up to the labels' discretion - all those that are available on demand on competing services would be available on Slacker, too.

Although Slacker does trump Pandora's catalog hands-down, the service's real competitors are not other "radio" applications, but on-demand subscription services like Rdio and MOG.

Here, the competition is a bit tougher.

How Music Discovery & Management Compares

With MOG, for example, you have access to features similar to Slacker - favorite tracks on demand, top charts, playlists and more. But while Slacker breaks down the charts into "top 50's" by genre, MOG offers "Top 50" charts for all music being played on its service, available by song, artist or album. (This is on its mobile app, to be clear).

The serendipitous discovery on MOG comes instead from a curated "Editor's Picks" playlist and "Featured Playlists" from other MOG users. There are also featured "Radio Picks" if you prefer the radio format over the on-demand format.

From the Web, you can favorite other MOG users' playlists or build your own and these then become available on your mobile. But in the mobile app itself (iPhone and Android only), you can only favorite a track or artist. You can't add a given track to a playlist on the fly, as you can on Slacker.

Meanwhile, on Rdio, the emphasis is more on social discovery of music through friends rather than through "editors" or strangers with similar interests. In a "recent activity" section on the mobile app's dashboard, you can see your friends' activity on Rdio, including music they've added or who they're following. This is a decidedly hit-or-miss experience for discovery of new tunes, however, unless all your friends like almost exactly the same music you do (which is never the case, we've found).

Rdio also features a "heavy rotation" section so you can see what's popular on the service overall, but this is more limited a snapshot than what MOG and Slacker offer. There are not multiple charts, by genre or otherwise.

On Rdio's iPhone app, a recommended music section has been added, offering a somewhat iTunes-esque "genius-like" list of suggestions based on the songs you've already played. Unfortunately, this same feature has not yet made it to all other mobile platforms yet. (Rdio is available on iPhone, Android, Windows Phone 7 and BlackBerry.)

However, Rdio users can save tracks to their "collection" (an online music collection built using the songs already in your iTunes media library), and they can add tunes to playlists from the mobile app. That means MOG is the only one of the three without playlist management from mobile. For what it's worth, Rdio's app is also one of the better designed streaming music mobile applications available today.

Catalogs! Who Has Your Tunes?

Slacker touts a catalog of over 8 million tracks, and Rdio says it has 8.5 million tracks. MOG, however, has recently bumped up its catalog to offer 11 million songs, making it the largest of the three we're examining today. (For comparison's sake, iTunes now offers 13 million songs, Napster has 10 million songs, Rhapsody, 10 million, Grooveshark, 6 million, Spotify, 13 million.)

All three streaming services say they offer songs from the major labels, however, so these shortcomings will only affect users with more indie or esoteric tastes. Much of MOG's indie selection comes from major indies (Beggars Group, Dischord Records, Matador, Thrill Jockey, Merge Records, Domino, Warp Records) and major indie aggregators (ADA, Redeye, RED, IODAlliance, INgrooves, Virtual Label Group, Finetunes, IRIS, CDBaby - self-released artists, and Tunecore - self-released artists).*

*Yes, those names only mean something to you if you're really into indie music.

Though MOG's app isn't the best in terms of its design, or even features (due to its lack of playlist management on mobile), the ability to stream the most songs may be its top selling point.

So what's Slacker's main attraction then? It doesn't have MOG's catalog or Rdio's deep social sharing integrations, but it does offer a lot of "top 50" stations - which is great for those who want to explore music based on popularity, instead of those who already know what they're after.

Depending on your mobile platform of choice, and the features that you value most - social, design, catalog or discovery - one of these three can easily fulfill your needs. If not, there are the others we mentioned above, plus iTunes of course...and torrents, we suppose, if you're naughty.