I'm sure you probably already understand the difference, but I will say that you should give some thought to your immediate goals. IMO, the CISSP is frequently identified as one of those MUST HAVE certs that you will need to have to even be considered for InfoSec jobs. OTOH, if you are already focused on Pen Testing in particular, the GWAPT might demonstrate more specific knowledge in that area, and could be more useful. I find that most organizations (HR/Recruiters) aren't quite asking for the GIAC certs, but they are highly respected by InfoSec professionals. The cost is roughly the same, so another thing you might consider is that you have already been staring at pen testing related materials in prepping for the CEH; that might make it easier for you to knock out the GIAC cert first. I don' think of the CISSP as difficult, but depending on your knowledge in areas such as physical security and policy, you might have to spend a bit more time studying those areas.
Well, now that I have thoroughly NOT made the decision simpler for you

, good luck and keep moving forward!
I still have to backtrack to redo my CCNA; I really don't want to be bothered with it, but I have to for work. I am working on my doctorate though, and really finding time for certification studies hard to come by.