As others have said, it really does boil down to personal preference and what really strikes you as necessary.
I don't care for the body lines on the S550, and IMO, the S650 went even further down the "I hate it" lines. There are some things to like about it, from a performance standpoint, but at the same time, there are things that go against it. Ford has added more power and capability, but they've also added significantly more weight, too. Also remember that with more technology, comes more electronics that can fail.
When I had my 2013 GT, I didn't think I was ever going to get rid of it. It drove well, performed well, and looked fantastic.
I traded it in for the 2014 GT500, because this car is everything I wanted my GT to be.