Here are the main differences:

1- Couples San Souci is a resort. Except for a possible day excursion, you were locked into staying on the resort. To be fair we knew this going into it, but realized as result that we truly don't like the idea of being locked into a specific place.
2- First and foremost compliant: the nude beach is small and really wasn't that nice. this is a nude beach not a clothing optional beach. There was one particular day and it got a little cold later in the day so my wife covered part of her body with a towel while lying on the beach. A guard came up to us and asked her to remove the towel or leave the beach. The guard didn't want to hear that she was simply a little cold. Although we're naturalists, this put a bad taste in our mouth and the policy seems a bit rigid. However, taking this experience aside there is just no comparison to Orient Beach and San Souci. Note, all other places on the resort clothing was mandatory.
3- The food at Couples was disappointing. I think we got a little spoiled in St Martin.... They pushed people to go to the buffet which in my mind is fine for breakfast but not something I want to do for lunch & dinner. There were only a couple of restaurants and reservations were needed and difficult to get. Last, the dinner attire was a little dressy. The overall facility was dated and needed updating and TLC (although I felt the rooms at Club O weren't nice either).
4- While the people were extremely nice & accomodating, culturally I prefer St Martin which is hard to articulate into words.
5- We've met some fantastic people at Club O over the years and just love the overall Club O atmosphere. There seemed to be far to freedom to do what you want and I never met an unfriendly person there.

Thus far, we've mainly been c/o beach people but would like to expand this a bit. We do enjoy going to dine in Orient Square & Grand Case. The food & culture is a clear draw for us. I guess if I had to sum up we're looking to be c/o most of the time except for meals (mainly dinner).