Sarah Perez over at ReadWriteWeb has a lengthy comparison of the cloud music offerings from Google and Amazon. One major obvious difference:
Like Amazon's Cloud Drive offering, Google's Music service is also being launched without the record labels' support. Google's failure to negotiate a proper deal with the labels led to the delay in the launch of Google Music, according to reports. However, in Google's case, this is a far worse problem than for Amazon because at least Amazon already had a (legal and licensed) online MP3 store where it sells music. Google does not. For end users, that's certainly a shame, but for Google's own purposes, it may not matter as much. Unlike Amazon, Google's main goal isn't to sell more MP3's to end users, it wants to sell Android-based phones. More philosophically, and core to everything Google does, its goal is also to get more people online, using the Web and Google services, all so they can see and click on more ads.
-ReadWriteWeb, May 10, 2011
I'll go way out on a limb here and say I think this is a bigger deal than Perez does.