I suppose the question is "How does CISSP-ISSMP differs from CISM certification?". Below are some of the differences:
ISSMP is not a standalone certification, rather it's a specialization exam for CISSP holder
CISM -- 5 domains (used to be 4)
CISSP-ISSMP 8+6 =14 in total
ISSMP requires additional year of experience in one of the 6 domains (on top of CISSP).
CISM covers Information Security Program Development and Management, something that does not get covered by CISSP or CISSP-ISSMP domains explicitly.
Given that you have covered experience requirements, it will take you longer to get CISSP-ISSMP than CISM (time needed for CISSP exam + time to endorsement + time to CISSP-ISSMP exam vs time to CISM exam)
CISM is less known than CISSP but CISSP-ISSMP has less visibility than CISM