Both.
Adding someone to the moderator usergroup does not make them a moderator, it only gives them some forum moderator permissions. To make them a proper moderator (giving them the staff banner, access to the moderation queue etc) you need to add them as a moderator in the ACP.
As I only have global moderators (moderate the whole site, not just specific forums) I add mine in the ACP but don't give them any permissions on that screen, I add them to the moderator group to give them these. That way if I ever need to give more permissions to my moderators (as I'm about to do with some new Xen Media Gallery Permissions) I can do it once in the moderators usergroup rather than to each moderator individually.