996 GT3 will keep quite stable, maybe losing something like 15000 Euro in two years or so if taken with a very low mileage (10000kms) but not more. Probably even less. A carrera S i think will lose more then that, especially because it's normally a lot more used. It's difficult to find GT3s with more then 40-50000kms on, even after 6 or 7 years, but it's usual to find Carrera S with that amount after only 2 years.