I don't think options come into play between the base and S. They are different cars - you make that decision primarily based on engine and budget. The difference between S and GTS is primarily the options and the front end. So key will be how likely you are to want a large portion of the GTS options in a car. If you would add enough of them to an S config, the break point becomes pretty easy to find based on preferences. That break point is then unique to the individual because there are elements (like the front end) that you can't really get on the S outside of the aftermarket so if you really like some of those elements, that would shift the break point.