We've been to CN, CSS and CSA and CSA is our favorite. We're around your age (34 and 35) and found that CSA is the best fit for us, and it seemed like more people around our age at CSA. If you're into fitness at all, CSA is top notch (the fitness center across the street is incredible). The beaches in Negril are amazing and we didn't like the beach at CSS at all. That's just us though. I wrote a very long review that is on this board from when we went in December and split time between CSA and CSS, with a lot of comparisons to CN because we had been there twice before. You can search through my profile for the post (sorry I don't have a link for it right here). That might help you in your decision.
As for splitting, we did, but not with such a short stay. By the time you get the feel for the resort, you will be packing up and leaving. We did 4 nights at CSS and 5 nights at CSA.
Hope you find this helpful!
Melea & Joseph
CN - '06, '07, '08
CSS and CSA - '09
CSA - '10
Bookmarks