Finally, the services have different rules on how many devices you can use to watch media streams or to download content.
In Detail: Amazon Instant Video and Prime Instant Video
Amazon's focus on selling content as opposed to hardware is obvious in the way it runs its media store Customers can watch Amazon Instant Video on dozens of supported devices, some of which--including the Kindle Fire, computers, and the XBox 360--let you choose between streaming rental videos and downloading them for offline viewing.
Customers who pay $79 for Amazon's Prime service, which offers free two-day shipping for all purchases, also get free Prime Instant Video access to streamed versions of some 13,000 movies and TV shows. Again, this service doesn't work with all Amazon Instant Video-compatible devices--for example, TiVo DVRs don't support Prime Instant Video.
Amazon lets you download purchased content to two compatible devices. If you want a PC to be one of them, you must install the Amazon Unbox Player. You can make backup copies of your purchased videos on a PC, and Amazon recommends doing so to ensure that you can restore the content.
Amazon does maintain your purchases in a so-called digital locker, however; as a result, if you run out of space and have to delete a downloaded video on, say, a tablet or a DVR, you'll likely be able to download it again. Amazon says that it can't guarantee that the content will be available indefinitely, owing to license restrictions "or other reasons."
Also, you may have to initiate your download online: Though I can buy and download content on my TiVo by going through a registration process and getting a five-digit PIN to punch in whenever I want to approve a purchase, I must go to a Web browser to download items I've purchased online, or to retrieve deleted content from my digital locker.
After downloading content to two devices for viewing offline, you must deactivate a license if you later want to download the content to a third device. Freeing up a license can be complicated--the process varies from device to device--and it may involve removing all licensed content, so read the online instructions carefully.
Amazon restricts renters to a single download or video stream, but it permits users to stream two different videos simultaneously to a single account--a nice feature if mom and dad want to watch one movie and the kids want to watch another. Rental terms--specifying how long you have access to the video, and the duration of the viewing window once you start watching--vary depending on licensing agreements.
The Kindle Fire doesn't support HD video, but it gives you the option of purchasing HD content in case you plan to access it later on a high-def screen. In my tests, the Prime Instant Video SD stream for Sabrina ran fairly smoothly, though not without occasional freezing and crashing.
Amazon makes its video content accessible in two formats: H.264/AAC within a .mp4 container, and VC-1/WMV9 within a .wmv container. AAC video is best for Macs, while .wmv is optimized for Windows Media Player.
Amazon handles music differently from video. When you buy music, you can either download it or have it stored online on your Amazon Cloud Drive, so you can stream it from devices with the Amazon Cloud Player. Amazon lets you upload your existing music collection to the Cloud Drive, to a maximum of 5GB of storage--and tunes purchased on Amazon don't count against that limit.
Amazon says that, where possible, it encodes MP3s using a variable bit rate, aiming at an average bit rate of 256kb.
You can access music on the Cloud Drive from no more than eight registered devices--and Web browsers count toward that total. In fact, if you clear your cache and discard the cookie Amazon gives you, the Cloud Player will count that sam browser as a new one the next time you use it. It's possible to unregister devices in your account settings, but you must call customer service to get rid of duplicate versions of a browser that count toward the device limit.
If you you want to download your music purchases, you'll need to install the Amazon MP3 Downloader to your computer. It is essential for downloading more than one file at a time, and it can import your downloads to either your iTunes library or your Windows Media library. But if you lose a downloaded MP3 file, Amazon won't replace it.
Apple iTunes Store: Biggest, But Not Best?
Apple pioneered the online media store when it launched iTunes, and the breadth of its content offerings is unmatched. But the features in the current version--iTunes 10--are starting to look a bit dated.
Most notably missing is support for streaming purchased content to iOS devices. When you buy a song or a video on your iPhone, iPad, or iPod Touch, you must download it to play it, and that can take a while. Apple TV does let you stream iTunes purchases instantly, however, and several third-party apps promise to add streaming support to iOS devices.
Otherwise, Apple--the only one of the three stores reviewed here that requires you to install desktop software--treats purchased music and videos fairly consistently. You can store iTunes content online using iCloud and sync it to as many as ten authorized devices, including five computers. De-authorizing a device from within iTunes on a computer is easy.
Purchasing videos from the iPad iTunes app requires only entry of your Apple account password. Once content finishes downloading, it appears in the appropriate player (music or video).
The New Kid: Google Play
The new kid on the media store block, Google Play doesn't yet have the volume of content (especially HD movies and TV) or ubiquity of device support enjoyed by its competitors.
But if you happen to own an Android device of fairly recent vintage, acquiring content via the Google Play Store app is as easy as doing so on the iPad or the Kindle Fire. Simply maneuver to the appropriate content category, and search or browse as you prefer. The Play Store charges purchase or rental fees to the credit card that you have on file for all your Google purchases; if you don't have credit card data on file yet, you'll be prompted to add it.
Music that you buy from Google Play automatically goes to your online Google Play library, and you can stream it to any Android device or browser. You can also upload up to 20,000 songs from other sources to the library via Google's free Music Manager software, which makes the service great for streaming your tunes anywhere. But you can stream music to only one device at a time.
You can also opt to download purchased tunes to up to ten authorized computers or Android devices for offline playback. If you run up against the ten-device limit, you can de-authorize a device and then add a new one--but you can't do this for more than four devices per year.
Download limits are less generous for movies and TV shows: You can download them to only five devices, and you must delete the download on one device if you wish to view it on a sixth screen.
This story, "Google Play vs. Amazon vs. iTunes store: how the content stores stack up" was originally published by PCWorld.