from what I heard, Carerra is more stable at higher speeds and GT3 is has sharper handling overall.
here's the Tuning Guide for the 3 Porsches
911 GT3 RS
Engine: Green
Turbo: Green
Transmission: Blue
Suspension: Green
Brakes: Yellow
Tires: Yellow
911 Turbo
Engine: Red
Turbo: Blue
Transmission: Green
Suspension: Blue (Green for highway races)
Brakes: Yellow
Tires: Yellow
959
Engine: Yellow
Turbo: Green
Transmission: Yellow
Suspension: Blue
Brakes: Gold
Tires: Gold
Which one should Carerra S Follow .. if any?
For those who needs an opionoin I found it here