I was originally going to take the CASP and then decided to just take the CISSP instead. They are similar, but totally different. They both cover a lot of the same information, but the CASP seemed more technical and focused on some of the gritty details, while the CISSP was more broad and touched on a lot of things, but not as granular.
Do I think it would be good prep for the CISSP? Absolutely. Food for thought: While the CASP is recognized as an IAT Level III cert, it only is classified as a level II cert for IAM. CISSP is still considered one of the "gold standards" for information assurance certifications. It is definitely still a good cert to have, but doesn't carry as much weight as the CISSP.