I think both of them serve the same purpose of securing the website. Although both of them serve the content of the website through HTTPS, when I check my website against some redirection checkers (Like this one: http://www.redirect-checker.org/index.php), only Always Use HTTPs will do the redirection [HTTP 301].
So “Automatic HTTPS Rewrites” will only apply if your server support HTTPS and you will need to have a valid SSL cert. in order for this to work. While “Always Use HTTPS” will just do it anyway, which might cause problems if your server doesn’t support HTTPS. Is my interpretation correct about what you said?
If you have SSL enabled for your site (I hope you do), then turn them both on. That way, your site serves SSL, and all links on your site that can be served with SSL are served with SSL.
Yes, I have SSL enabled on my site and I have turn both of them on. In addition I also have the CF Origin Certificate installed with “Authenticated Origin Pulls” checked.