There are significant differences in the histories of the CNS and NP roles which lead to different foci. However, some people would like to see the roles combined and minimize those difference.
The history of the NP role involves the need for nurses who could function in a "physician extender" role in outpatient settings. While the NP role has grown and further developed over the years, most NP students spend a lot of time learning how to provide primary care to patients in outpatient settings.
The history of the CNS role lies in the need for Master's prepared specialists to support nursing practice in hospitals -- to be clinical experts in traditional nursing care to serve as resources for staff nurses, provide consultation and expert advice regarding the care of the most complex patients, to develop nursing policies and procedures based on scientific findings, assist with staff development, etc. CNS students learn how to be successful change agents, teachers, etc. within health care organizations and focus on the care of hospitalized patients more than the provision of primary care.
Many CNS students spend little or no time learning primary care -- and many NP students spend little time learning how to teach, manage, or work within the hospital system. If you look closely at various academic programs, you will see how some programs emphasize inpatient care and the completion of "change agent" projects while others have long hours of clinical focusing on primary care in outpatient settings.
Which type of work interests you the most? If you want to work in a hospital and help staff nurses provide the best nursing care possible, then a CNS role is probably the right role for you. If you want to work in an outpatient setting and focus on providing primary care, then an NP role is probably right for you.
Of course, there are exceptions to what I have said. Some programs try to merge the 2 roles ... as do some employers
. But this gives you a place to start as you examine different jobs and different academic programs.
Good luck with whatever you decide.