When it comes to digital music, we now have a wealth of services choices, but also a wealth of services types. There are the traditional online music services, like iTunes, which offer individual tracks and albums for sale. And there are the digital radio services, like Pandora (and related services like satellite radio), which provide music lovers with dynamic playlists based on favorite music genres, artists, or even songs. And, finally, there's a third choice: Digital music subscription services, like Rhapsody and Zune Pass, which let you access a library of several million songs as if it were your own, for a monthly fee. These services split the difference, if you will, between the first two choices.

Pundits, including yours truly, have long argued that the latter type of service, based on a monthly subscription fee, would ultimately win out. But that hasn't happened because of compatibility issues--each service is limited to PCs and certain mobile platforms--and high costs, and simply because the biggest provider of music, Apple, hasn't yet opened up to the concept. (One has to think that an iTunes-based music subscription service would be hugely successful.)

Music lovers firmly in the Microsoft camp should at least examine Zune Pass. It costs $15 a month, which sounds extravagant, but unlike other similar services, it includes $10 worth of downloadable, unprotected MP3 files (i.e. 10 songs) each month as part of the subscription. So it really costs only $5 a month if you're actively buying music anyway. Zune Pass works with Windows-based PCs (via the excellent Zune PC software), with any web browser (via Zune.net, providing Mac compatibility as well), with Zune HD (and older Zune) devices, and with all Windows Phones.

But Zune Pass falls apart once you start to look outside Microsoft's insular and small digital media ecosystem. So if you have an iPhone (or iPod or iPad), an Android handset, or any other non-Microsoft device, Zune Pass is basically worthless. That's a shame, and it makes me wonder why the software giant hasn't strived to bring native Zune clients to these other platforms. But they haven't, and in ignoring these huge markets, it has opened the door for nimbler, more open competitors.

The biggest of these, oddly, is not Real's Rhapsody service. Nor is it Napster. Instead, it's a service called Spotify. Previous to last week, Spotify was available only in several European nations, which sounds like a recipe for irrelevance. But Spotify has garnered a whopping 10 million registered users in Europe alone, 1.6 million of which are actually paying the company a monthly fee to use a higher-end version of the service. So the feeling has always been, when Spotify comes to the US, it's going to really take off.

I've been using Spotify since its US debut last week, and as a long-time user of competing subscription music services, I can tell you that Spotify doesn't really bring anything new to the table. In fact, it's missing some features from Zune Pass, in particular. But the important thing to remember when choosing an online service is that it's not just about the service itself. It's about whether it's supported broadly and popular, and likely to continue forward. You don't want to waste time or money with something that's going to disappear in a year or stop growing. And on that note, Spotify is likely a better bet than any of the competing subscription services.

The other important bit to consider is that most people haven't ever tried a music subscription service, so those who do wander into Spotify's camp will be experiencing something that is new to them. And what they're going to find there is actually pretty decent.

Spotify offers roughly 15 million songs in its collection, a figure that is in the iTunes ballpark and ahead, I think, of Zune Pass. The service is fully supported by all of the major recording companies, which is actually a pretty big deal, as it means there will be fewer content gaps than with some services.

Spotify offers three versions of the service. The first is free, and PC-only, providing streaming, ad-supported music with certain limits on the number of times you can play the same songs in a given month. The mid-tier service, Spotify Unlimited, costs $4.99 a month and provides unlimited music streaming with no ads. And at the top of the heap is Spotify Premium, which for $9.99 a month adds mobile device access with offline mode (essentially the ability to download tracks to a device or PC) and better streaming music quality. (That said, the quality of Spotify Unlimited music was excellent in my own testing.)

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Spotify for Windows

Even the free version of the service is of great interest, since you can use it as you would Pandora or other digital radio services, and augment an iTunes-based music collection with a more diverse set of music. But when you move up the pricing tiers, the capabilities expand, and I still think a lot of people would prefer to access a several million song collection of music, creating unique playlists, rather than buy and manage their own music.

If you do opt for the free version, you're pretty much on your own for music management. You can search for an artist, song or album, and add that to your Play Queue (what other players called Now Playing) on the fly. You can save playlists you create in this fashion, discover new music that's similar to what you've search for, share on Facebook, Twitter, or Windows Live Messenger (a more open take on Zune Social), read artist biographies, and so on.

Step up the ladder and the real power of Spotify becomes apparent. You can sync music to devices (like iPods and Android phones) or just download mobile apps for the top platforms (iPhone, iPod touch, Android, Symbian, Palm webOS, and Windows Mobile, with Windows Phone on the way) and access the music on the go. (I opted for the Ultimate service since Windows Phone isn't supported yet, so I haven't tested mobile device access.)

Unlike some services, Spotify actually provides a very iTunes-like native app for Windows (and one for Mac OS X) that lets you access locally stored files as well as those in the cloud, and move music back and forth between Spotify, iTunes, and Windows Media Player. Playlists are saved to the cloud, so when you access your account from any PC or device, everything you've created is available from anywhere.

To the jaded Zune Pass subscriber, most of this is old news. But Spotify is more open than Zune, with better compatibility and sharing via the services people actually use, and not Zune Social. This is a crucial distinction, and until and unless Microsoft opens up Zune Pass to the iPhones and Android devices of the world, it's always going to be an also-ran service. Spotify thus has a chance to become the iTunes of the music subscription world. You know, until Apple just does it. You have to think it's at least in the planning stages.

Until then, you could do worse than Spotify. The only upfront issue is that the free version of Spotify requires you to hop on a waiting list for now. (If you opt for a paid version, they'll let you in immediately.) Music lovers should sign up. It's not particularly new or innovative, but this just may be the future of digital music.