The Security+ is a lot deeper in certain areas than the CISSP. I used the Security+ in order to work on my MCSE: Security, and I got it during the time I was studying for the CISSP. I found that some of the stuff on the CISSP helped however the Security+ certification seems to almost assume that you have the knowledge of the Network+ stuff because all of the review material was much more focused on the technical side of life rather than the policy side of life like the CISSP. While I know that folks will probably frown on this, I studied for the Security+ using the Exam Cram book because I'd already been doing this stuff for a while and I wanted to figure out where I was lacking knowledge and fill in the gaps. If you are studying for the CISSP and have some real world experience, that may not be a bad way to go, and then the areas that you are missing you can probably find good references on the internet to augment your knowledge.
CISSP, CSSLP, MCSE+Security, MCTS, CCSP, GPEN, GWAPT, GCWN, NOP, OSCP, Security+