Difficult one to call. The Apr-Jun offer was better than the July-Sep one so if the trend continues....Also they must have shifted a he'll of a lot of C63s over the last 6 months so they may be easing off, but then they may see December as slower and nee to push harder. Sometimes the change in figures is only as a result if dropping the GFV slightly.
Either way, if you ordered during Sept but the car is coming in December, I would push for whichever deal is better for you.
S