We have been to both in the past 2 years. These were our impressions from the times we spent there. I'm sure the crowd varies from week to week.
CN has a better beach, is more compact, more casual, less personal and intimate, and has more groups of couples traveling together and partying together. We did not really get to know any staff at CN. Diving was slightly better at CN. The cat cruise is fun at CN and not available at CSS.
CSS is more beautiful, more romantic, more personal and intimate, quieter, with the feeling of a tropical rain forest at the beach. It seemed to be mostly individuals couples traveling alone as couples. It was easier to get to know the staff and they were a bit more interactive with guests. It also has a more formal and elegant atmosphere in the evenings. We also got to know more other guests.
I personally think that all the "group traveling" couples take something away from the Couples atmosphere, and CSS clearly had less of this.
The guest ages seemed about the same at both resorts when we were there. Food was also similar and very good at both.
For us, we easily preferred CSS for the above mentioned reasons, so it really comes down to what you are looking for. Good luck with your choice.