I took CISSP before GSEC. I still haven't done GSEC, and probably won't get the opportunity for at least a year. The reason (misguided as it was) was that i was thinking I could use the study time for the GSEC towards the CPE credits needed to maintain the CISSP. (since I wanted both anyway) The reason I say it was somewhat misguided is that yes, that is certainly worth some CPE credits, but what I had not known is that I would not have any trouble whatsoever coming up with enough credits anyway. CPEs, I was told over and over again, were a pian in the butt and hard to come by with current budget constraints. That is pure BS. I have had my CISSP since June 2009, and now, not even a year later I've already got more than my 3 year renewal's worth of CPEs. I don't bother to record everything that might count as CPEs.
Take the one that interests you more first. That said, I learned a LOT preparing for the CISSP and I am very glad I did it.