Don't know of a chart, but I have 105GB on Dropbox and a full terabyte on Google Drive and use both extensively. Drive is getting better and fully integrates with Google Docs and other Google services if you use that stuff. But Dropbox is faster and more flexible for ordinary files. Plus if like me, you use Linux, Dropbox is the only cloud storage service that offers a real Linux desktop client.
Registered Linux user #266531 since 2001. Android user since v1.0.