dr_hfuhuhurr, stiffer is generally better when you are running foam tires on carpet. A stiffer chassis/upper deck makes the car handle much more consistently. Which leads to a tigher, faster line and fewer mistakes. Also, it seems like a stiffer car carries more corner speed, which is what roadcourse racing is all about.
A more flexible chassis/upper deck is better for low-traction racing, like running rubber tires on either asphalt or carpet. The extra flex gives more traction to the car, when it would otherwise slide in the corners.