I'm leaning towards 'official' myself, provided the demo was 'released' by the artist them self. By 'released', I mean anything from a full commercial or promotional release, to the disclosure of material in any format to a single person outside of the recording/performance process (usually a fan).
Does promo not have a more specific definition, i.e. a release distributed for radio play etc or to encourage the purchase of another release? In the case of releases essentially for fans, and in neither of the above cases, 'promo' doesn't seem right.