I'm not sure whether I would pick FTPS or SFTP as the superior protocol for automated file transfer (HTTPS is rather poor, although HTTPS-to-S3 is getting close

I'm not sure whether I would pick FTPS or SFTP as the superior protocol for automated file transfer (HTTPS is rather poor, although HTTPS-to-S3 is getting close). On the one hand, SFTP has built-in support for key-based security (client authenticates with its own key, and checks the server's key), but the protocol itself is a mess that pretty much assumes the server is a UNIX box with a typical file system. We had the interesting challenge of writing a windows-based SFTP server that stores data on Azure Storage, and even the most surprising things (like one command to create an empty file, followed by another command to upload to that file) turn into a silly mess, not to mention the difficulty of setting up an SSH server on Windows. FTPS gets those things right: security is based on SSL (more implementations available) and most file-altering operations are done with single commands. On the other hand, it's still password-based (although encrypted) and the data-channel-versus-control-channel problem becomes a mess with load balancing.