Read the many reviews and use the search feature to find the many threads comparing the two.
Both meet your criteria.
We prefer CSA for the large terraces on all rooms as well as rooms in the original section. We also prefer the additional dining options, including ala carte options for breakfast and lunch. Better nightlife at CSA based on our experience. And the sports complex is fantastic.
We also really enjoy CN - pool is better there and the resort's beach is more private if that matters.
Having been to both, CSA definitely offers up more nightlife and more "fun" since it has the Aura Lounge where you can dance and have fun until the wee hours of the morning. The piano bar at CN turns into a "nightclub" one or two evenings a week and is way smaller than the Aura Lounge.
Both resorts are gorgeous, have amazing beaches, and both are relaxing. Although to compare, CN has a more relaxing atmosphere on the beach during the day. There are fewer vendors walking by and fewer people in general because the resort is smaller. There is also the au naturel section which is separate from the main beach so many people head over there.
CSA had much more space on the beach before last fall's tropical storms blew through. However, there are still plenty of loungers for all!
We started going to CN when I was in my forty's, now I am in my very, very early 50's and I am counting the days to our trip back home to CN, if that helps any. (as a side note, we were married at CN and honeymooned there as well, on our third trip)
Well, I will be 2 months shy of my 40th birthday when my fiancee and I get married at CSA next May. He will turn 42 one month before the wedding. I had to go with it for the size of the resort and the variety of things you can do. From the advice I've gotten, at CSA you can party with other couples or opt for romantic seclusion. We like the option of doing either. Plus, we're both powerlifters and can't stay away from the gym for nearly two weeks--CSA has the best sports complex, hands down.
We've been to CTI once, CSA once, and CN...12 times. We started in our 30's and are now in our late 40's. Lots of reasons, but we really really like the privacy of the beach relative to Swept Away. The beach at CSA is much more of public beach - lots of traffic and vendors. We also use the a/n beach at CN, pretty much from sunup to sundown, Bloody Bay is just beautiful (and great for my limited sailing skills), we like how CN is laid out better (pool and food and entertainment is central, unlike CSA which is spread out).
We were at both CSA and CN last year. I liked them both. During my visits, it seemed that CSA had a younger overall crowd than CN. I am in my early 50s. There were a lot of people in their 20s who seemed to be on their honeymoon. I think CN is more relaxing. As someone noted, a storm last fall washed away about half of the beach at CSA. A guy that worked there said it would replicate itself in a few months - don't know myself how that works exactly. The beach area at CSA I found to be crowded. It was hard to find a place under the shade. Plenty of chairs - just all the spaces were taken in the shade. We did not have this problem at CN. I don't think you could go wrong with either of them. They are both great resorts!