The CL63 turns as many heads today as it did when it first came on the scene in 2007. It looks fast, is expected to be fast, and stands out in a crowd of front row valet parking. The CL is less than 8% of Mercedes production and the CL63 almost doesn't register on the charts.
The S63 turns less heads, is fast, but isn't expected to be fast. The four door sedan garners respect and admiration from the more knowledgeable aficionado. The S63 still qualifies for valet prominence but it can still be easily confused by many as anything from an S350 to an S600 (present company excluded).
The CL63 is an SL for big guys who very seldom need to fill the back seat with people over 5'2", don't mind spending an extra $20k for 2 less doors and enjoy standing out in the crowd. The S63 makes daddy feel like Michael Schumacher sitting in the mall parking lot waiting for the Mrs, need or appreciate the extra 8 inches, and savor the thrill of dusting overzealous M3's with 2 1/2 tons of Mercedes at it's finest.
Obviously, this is purely my opinion.