We went to CSA for our honeymoon and had a great time. The beach is wonderful as well as the fitness area. Also there is a fun sunset cruise that you can't get at CSS.
We then went to CSS for our 1st and 2nd anniversary and loved the romantic vibe of the resort. The porch we've gotten both times was a lot bigger than CSA and we loved the jungle feel of the resort. The beach isn't as nice as CSA but it's not horrible either.
We likely will go to CN for our 3rd anniversary (see a theme here?) to check it out. I recommend going to the couples message boards and snoop around to help you decide which resort to pick. No matter which one you pick you'll still have a wonderful time.
We've been to CSA and CN. The room (beachfront suite) was far superior at CSA but we preferred juts about everything else at CN. The beach was more private and felt less crowded and the resort was less spread out and felt smaller/more intimate. Couples food is phenomenal for an all inclusive. We also love the adults only, couples only aspect. We have plans to visit again, probably CSS this time so that we can also check out CTI.
We did CSA for our honeymoon and it was amazing. FYI there is a very active message board on the official couples website. I read that board every day before our trip.