With Amazon I can pay $20 per year and receive a limited special offer whereby I can upload any amount of music that is already on my computer without it eating up any of my 20GB storage space.
The alternative is to use Google Play, which allows me to upload up to 20,000 songs completely free.
In iOS there is an Amazon Cloud Player app that will let me stream my music to my iPhone. Right now, there is no Google Play/Music app that allows me to do the same thing. I can, however, browse to the Google Play website and play my music on my phone that way.