Here’s the premise for both Amazon Cloud Player and Grooveshark: you can upload your favorite songs to the cloud and then access them on-demand from anywhere. Once your songs are in the cloud, you can play them from any computer, laptop or netbook or Android smartphone or Android tablet.

Both web apps are incredible. They do what they say and they do it intuitively. But there are some subtleties to how these services are delivered that make one of them a better place to store your music than the other.

It all boils down to legality.

So far, Grooveshark is losing big time in this department. While there aren’t yet any indications of an impending shutdown, Grooveshark has been a lightning rod for performance rights suits from record labels. That’s because when you upload songs to Grooveshark, they become available to everyone. Anyone can upload any file that they choose, and as far as making sure that making music available via streaming to the world doesn’t infringe anyone’s rights, well, that’s all based on the honors system. Grooveshark’s terms of service attempts to indemnify itself from the actions of anyone who knowingly uploads copyrighted material. It should come as no shocker that this has done little to stem the flow of unauthorized uploads. Because of this, Grooveshark has become an interesting mix between Napster, Pandora and Lala (RIP).

Grooveshark’s master plan for all of this obvious abuse of its system is to offer record labels and artists with gripes two options: file a DMCA takedown request (which will be honored, given that all hoops are properly jumped through) or cut a royalties deal after the fact. Grooveshark has thus far been relatively successful with this strategy, and has fended off at least a couple big name lawsuits.

The problem is that the legal troubles for Grooveshark are far from over. And most of the big wigs are treating the platform as if it were radioactive. Both Google and Apple ousted Grooveshark from their app stores, citing that the application violated their terms of service. While its arguable that the companies have ulterior motives, the legal gray area that Grooveshark occupies makes it an unsafe bet for both developers and users. Grooveshark may be convenient for now, but if the service goes away, so too goes your meticulously cultivated library.

Now, Amazon Cloud Player is fundamentally different. While it may seem similar, there is a key difference in the way that your files are handled. With Amazon Cloud Player, when you upload files, only you have access to them. Amazon isn’t broadcasting copyrighted material—it’s merely serving up the content that you already own. The exception to that are songs purchased from the Amazon MP3 Store. These songs can be served up in a centralized fashion because Amazon has already made arrangements with the record label or artist for digital distribution of the song.

For the user, this behind the scenes treatment of your digital uploads is barely noticeable. True, you don’t have the ability to discover music with Amazon Cloud Player, nor can you access music that you haven’t purchased or uploaded. But what matters is that, given Amazon’s less tenuous legal strategy and undeniably more respected stature, it’s unlikely that your music collection will be vaporized due to legal pressures. And when you’re using a cloud service as a backup service for your songs and music, that matters.

Bottom-line: Enjoy Grooveshark while it lasts. But for reliability, choose Amazon Cloud Player.