A LDAP connection for the user management would be fine. To manage an AD security group is much easier as to manage seperat accounts with different passwords. Our Barracuda appliances are not in the Barracuda cloud (Firmware 6.0.05) and it is a pain to manage the user accounts for each appliance. Also from the security aspect it would be much better to use LDAP, because of password changes etc.
Single-Sign-On could be also interesting in the framework of LDAP.