Road or race?
My pick would be the M3 CSL
Same here, M3 CSL every time.
Reason is this: it is the perfect car to learn how to drive a touring car, and prepares you for the faster racing versions of these machines.
In my opinion Gran Turismo is and has always been more about the Touring Cars (both production and racing versions) than the LMP/Group C cars, just compare the sheer amount of them (DTM, JGTC, endurance touring cars, WTCC and so on) to the prototypes. They require much more driving skill because you can not just chuck them about and then simply make up time on the straights. You have to understand how and why these cars understeer and how you can tame that/use it to your advantage by changing your driving style or learn to customize a set up.
The CSL has,when tuned up, just about the same power level as a "stock" racing car, about 500 BHP, and is that little less agile which forces you to learn how to drive properly. And as it is (barely, but still) a production car, you can use it for most online/offline races and be competitive if you understand how it works. So I believe it would do it justice to have it as a premium, as does the fact that it's just gorgeous.
Sorry for the long answer, but you asked "why?"
