I stayed at both this summer. Andaz is much better. Nicer property, great facilities, and generally has higher room rates so you would be getting much better value. Unless you have a big reason to be near Roppongi I would choose the Andaz as a no brainer. In either event it is a very cheap cab ride between Toronomon Hills and Roppongi, and I can't see why you would prefer to stay at GH other than location.