Which Certification in Network Security is the most employer demand? CCNP Security or CISSP ?

CISSP is a broad cert. I would recommend you to go with a specialized cert/path such as : CCNA > CCNA Security > CCNP > CCIE  , BTW, You can just jump to the CCIE level by sitting for it's writing and lab exams directly as it has no pre-requets if you have a good real knowledge within the network security arena. All the best.