I would say it depends... Google play is a great service as it will allow you to store 20,000 songs for free. Amazon also has a decent music cloud storage. I use both but I primarily stick to the google play because I subscribe to their unlimited streaming service. With both service's you can install apps on your computer that will upload songs in the background, and both have players that give you access to your music on android and iOS. In both cases, digital music purchased from the site wont count towards your cloud space, however with Amazon, you get the same benefit if you purchase the physical media as well. So if you purchase an LP or a CD from Amazon, they will automatically add that album to your cloud storage for immediate use while you wait for the album to come in the mail, and again it doesn't count towards your could storage. In the long run, if you like to collect physical media, this could save you a ton of time.
So if you are into physical media, CDs or LPs, I would strongly recommend Amazons cloud storage service. Other wise I would try out Googles. Or if you cant decide... do both ;P