The design tolerances are not the same. Razors can change even with the slightest mm difference. That can affect exposure, rigidity and the overall performance. (And more)6C weights 100gr and 6S weights 116gr. The 6S is matte. The design is the same. The 16grams and the matte finish makes the 6S that better?
I feel a considerable difference between those two and I praised my 6C a lot.
I don’t dislike my 6C now, but like I said, the 6S is the real deal.