A belt drive will be capable of carrying far more power through a turn than a shaft drive, but, if power is scarse (stock motor) then a shaft will accelerate better. Once you start putting power in a shafty, under acceleration, the chassis wants to twist (remember, for every reaction, there is an equal and opposite reaction), so with a shaft drive, the car wants to twist with a belt drive, it makes the rear squat, putting more power and more stability through the chassis.