Yes, SSH and SFTP use the same username, password and port. Can you do SFTP from Terminal? (sftp email@example.com)
In Transmit, do you have any paths set for that Favorite (bookmark)? That shouldn't matter, but SFTP and FTP do differ in what's available path-wise. My Remote Path is /home/USER/DOMAIN.COM
And make sure the Protocol is really SFTP and not one of the FTPs with SSL.