SFTP client for OS X for updating external website
I use Transmit from Panic. It doesn't allow an actual file to file content comparison like a diff, but you can do a simulated sync to see which files will be replaced or deleted.

If you drag one file from window to window it asks you to verify and gives you the information on both files regarding modification dates and times.

You can try it before you buy it. I would think any good FTP client should do a simulated transfer.