File conversion: Both Google Music and Amazon Cloud Player give what they get. If you upload a 128kbps copy of The Beatles "Love Me Do," that's exactly what you'll get back. With iTunes Match, if a track from your library has been matched with a track available from the iTunes Store, it will come back to you as a 256kbps AAC file. In cases where your copy is of lesser quality, you win. However, if your copy is uncompressed or in the Apple Lossless format, the version you receive from iTunes Match will be that same 256kbps AAC format.
Access: Content stored via iTunes Match can be accessed within iTunes on both a Mac and Windows PC as well as from any iOS device (including the Apple TV 2). You can additionally download copies of your iTunes Match content to your computers and iOS devices (though not to the Apple TV--it can only stream this content). You can use iTunes Match with up to 10 devices authorized with your Apple ID.
To play Google Music and Cloud Player content on your computer, you must use a web browser. Each service presents an interface similar to iTunes where you can view your music by song, artist, album, and genre. Play controls can be found within these windows. Google Music doesn't provide a way through a web browser to download tracks. Google's Music Manager application can download tracks you've purchased from the Android Market, but it's an all-or-nothing process. You also use this application to upload music to Google Music. Each of these services allows you to use up to eight devices (computers and Android devices).
Within your web browser, Amazon Cloud Player does let you download tracks you've previously uploaded. You select those tracks within the web browser interface and choose Download. When you do, the Amazon MP3 Download application you've installed launches and downloads the tracks.
There are a couple of iPhone apps that allow you to access Google Music tracks--IIS's $2 gMusic and Sokolware's $1 GoMusic for Google Music. gMusic lets you stream your music to your iOS device. GoMusic allows you to stream as well as download it to your device. GoMusic's downloaded tracks are available within that app but are not made available to the Music app.
At one time, there was an app for accessing Amazon Cloud Player music but it was pulled from the App Store at Amazon's request. You can, however play your Cloud Player tracks within Safari on the iOS device but you can't download them to your iOS device.
Shaking it out