To me, it all comes down to looks. I own a S6 and have never gotten it so close to my face that I hit myself with it. But that could be just me.
I have played with a S6.5 for a day, a friend used my S6. After a game or two, I forgot which marker I was using. Jump back to mine, it felt a little funny but only for a few minutes. With a 45/4.5k tank and a 100 round hopper on top, I really could not tell the difference. Again it could just be me.
So then it just comes down to looks. Which looks better to you? But at the end of the day, you can not go wrong with either if it says CCM on the side.