I would take the manager's job. But then, that's why I'm not an admin assistant: I don't want to be one.
Are you enjoying working as an admin assistant? If so, why do something you don't want to do?
Are you considering moving jobs in the near future? A managerial post for 6-12 months will improve your CV and make it easier to get another job.
Really, nobody else can decide for you -- only you know what you want.