Where to Go from Here
Once you have received the CCNA Security designation, you are well suited to pursue the CCNP Security certification. Logically, the next topic to study would be for the SECURE exam, since securing an IOS routers, configuring VPN and Layer 2 security, are fresh in your memory from having done the IINS exam.
If you've not yet attained your CCNA certification, it makes more sense to start with CCNA before considering the CCNA Security exam. Why? CCNA Security requires a CCNA certification and the CCNP Security requires that you be CCNA Security. So, it all starts from the CCNA certification.