That is a TOTALLY subjective question:
What's nicer: chocolate or vanilla? Rocky Road or Butter Pecan?
Personally, I like Vanilla and Butter Pecan.
I like a firmer mattress and soft pillows.
I like the rooms better at CSA.
I think the beds are spectacularly comfortable (at all 4 resorts) and sleep like a baby the entire time I am there. (That could also have something to do with sun, fun and rum -- not necessarily in that order.)
At all the Couples resorts, you're going to get a nice, comfortable room. Nicer depends on what you PERSONALLY like. Look at the pictures. Which ones LOOK nicer to you? Do you like bright, more modern, eclectic tropical? Go with CN. Do you like dark woods, white linens, lush tropical? Go with CSA. The rooms are pretty much as they are pictured on the website, so if it looks nicer to you, it probably is.
And on the beds? Well.... have a nightcap.(or three -- it's included.) I promise, even if you don't like the beds, you won't notice.
We like the atrium suites/beachfront suites at csa. Those rooms are just the ideal thing for us. We love the wood and the shutters. We also love that they don't seem like a hotel. They don't share a wall with any other rooms so that is nice. There are windows on all sides. I would love to try another one of the resorts but I love those rooms so much and there really isn't anything else like it at the other ones.
Look at the pictures of each resorts rooms. It says it all there. CSA is classic clean Caribbean with lots of wood and white linens. The bed is very comfortable and the verandah rocks! At CN, the rooms look like the typical tropical room with bright colors and lots of white. I'm sure the beds are similar to CSA. Good luck deciding, but really, just look at the pictures.