How are salivary and plasma cortisol different?

Are salivary cortisol levels a more accurate indication of the amount of cortisol in use by the body? Is plasma cortisol the inactive precursor? If someone drinks a cup of coffee, how are cortisol levels affected and would it raise both salivary and plasma levels or just one (or none, if I'm really off-track here) ? If someone suffers from adrenal exhaustion and takes cortef (prescription hydrocortisone), would there be greater benefits if chewed or absorbed sublingually, thereby immediately raising salivary cortisol levels (?), or would it be better to be swallowed to raise plasma cortisol levels, to then be converted to an active state (another umption)?