If I understood @Karsten Iwen first reply, get both might be the best approach.
If you only can or desire just one, that's a hard call because it depends on the direction you want your career to follow and "insight" of your employer.
If your employer isn't insightful, having a particular certificate is just a item on their checklist for a specific position, i.e. a resume filter.
For an employer with insight a certificate can be used to provide insight about you, the person. For example, if you get just an Enterprise CCNP, that's good, but just shows your much like every one else. However, if you acquire a security CCNP, and that's NOT your job history, it demonstrates both your sincere interest and willingness to learn.
If you were only to obtain one CCNP, I would suggest going for the security variant. Ideally, you job history shows your experience networking, and a CCNP in anything provides a certain knowledge level in networking, and in this case, a security variant shows knowledge in an area outside of your job history.
Perhaps an interesting story of an insightful employer.
Early in my career I applied to a company, who the manager that hired me, and I worked for, later told me when he read my resume he thought this person was very bad or very good.
I asked why, and he said because at the several years you worked at your prior employer, your job role significantly changed every 3 to 6 months. So, either they were trying to find something you could do, or they were trying to figure out how to best use you.
I asked, how did you decide which it was? Oh, that was simple, I spoke with your earlier employers, and they wanted you back. (Note, however, this company didn't have a HR department to filter candidates.)
