I like Dropbox's binary diff sync as we don't need to upload the entire file when there's a minor change, especially useful for large files. But Dropbox doesn't support end to end encryption which is privacy concern. So, I wonder if there is a cloud service that has both ?
To get end-to-end encryption it is inevitable to install a piece of software on your client. Such tools exist and often work with more than one cloud. I would recommend to have a look at Boxcryptor, as far as I know it also supports differential synchronisation.
The sync scene (say that 5x fast) and sync software services are getting better and better everyday. I sometimes use Binfer for certain clients who are leery of cloud based sync systems. https://www.binfer.com/