Daniel Miessler has posted a pretty good breakdown of the differences between the GSEC cert and the CISSP cert. He is coming at it from the aspect of which one involves more technical savvy.
Without a doubt the GSEC is the more technical of the two. I have both, and the GSEC was infinitely more technical and required much more study and dedication. Yes, it is open book. But you try to read through those books and find all the correct answers in the time allotted (of course, if you have the old PDF files, you stand a better chance).
The CISSP was difficult just because of the sheer magnitude of it and the number of different areas you have to have knowledge of. But as Daniel says, you can cram for that and then forget it. The GSEC really does not work that way.
So for the most part, I agree with Daniel’s post in its intended premise, which is that GSEC is more technical in nature. However, both serve different purposes, so I believe both are valuable.