Apple has finally entered the music-in-the-cloud arena with the introduction of iTunes on iCloud, making it a competitor to Google's Music Beta and's Cloud Player. Here, we compare all three services. We'll update and amend the chart below as we get more information.

Music Beta by Google Amazon Cloud Player iTunes on iCloud
Storage capacity 20,000 songs. All songs need to be uploaded to Google's servers. Unlimited for Amazon purchases. For songs that weren't bought from Amazon, the limit is anywhere from 5GB to 1TB (that works out to around 740 to 152,000 songs, assuming each song is around 4 minutes long recorded at 255Kbps). Songs need to be uploaded to Amazon's servers. Unlimited for iTunes purchases. Everything is stored in the cloud. If you subscribe to iTunes Match, songs not bought on iTunes are "scanned and matched." You only have to upload the songs that can't be matched. There is a limit of 25,000 songs for songs that are not purchased on iTunes. A separate 5GB cap applies only to documents, mail, and backup.
Cost Free and invitation-only for now. 5GB storage for free; $20/year for 20GB and $1000 a year for 1TB. Songs bought on Amazon don't count against the limit. Free for iTunes purchased music. Apple offers an iTunes Match service that will match up non-iTunes songs with its library and upgrade them to 256kbps AAC DRM-free, and the iTunes Match songs are shareable with up to 10 devices. However, iTunes Match is a service that requires a $24.99 annual subscription
Streaming Yes Yes No; download only
Offline options Recently played songs are automatically cached for offline listening on Android devices. You can also manually select songs/albums for offline listening. But you can't download songs to a different computer. You can download the songs to a different computer/device without a problem. Any song can be downloaded to any authorized device at no additional charge. There's also an automatic download switch so any song bought on iTunes can be pushed to your mobile devices. iTunes Match songs are shareable with up to 10 devices.
Free music Google provides some free samples during initial setup Yes; some songs are offered for free occasionally. Yes; some songs are offered for free occasionally.
Store None Yes Yes
Mobile Android app; playable on iOS via the browser Android app; playable on iOS via the browser Integrated into iTunes on all iOS devices.
Requirements Google account Amazon account iTunes account
Other features Custom playlists that can be synced with the cloud, intelligent mix Amazon's Cloud service extends beyond just music. iCloud extends beyond just music into documents, photos, mail, calendar, and contacts.