With a forte already established within the Development arena the CSSLP would definitely be right in your wheelhouse. Depending on where you want to go with your career other Certs could have value for you to move in that direction. Assuming you are sticking with Development at some level the CSSLP would be the top recommendation.
I hold a CSSLP certification and found that it provided me with a good foundation in SELC principles. That being said if you are planning on working on a DoD contract you might want to consider the CISSP since it shares many of the same domains as the CSSLP and would qualify you for performing more IA functions. Check out the link below for DoD approved 8570 baseline certifications.
I received my CSSLP at the beginning of the year. I considered going for the CISSP but did not feel I had the proper years of experience in enough areas to qualify. The CSSLP I feel introduces the terminology, regulations, and how to integrate security in each area of the lifecycle. I feel like the CSSLP is the beginning of the process. After that comes digging into regulations and material to show how to counteract vulnerabilities and determining if your company has any guidelines on development security. One word of warning on the CSSLP the training material and the actual test questions are different. The questions in the training material are more definition/regulation based and for me on the test were scenario based. Also if you have not already done so if you work for a company engage your supervisor so that he/she can offer any department/company advice.