I think it depends on the weight of the rider. I've ridden two prototype boards, long effective edge (BX style) boards that were really soft. Designed to get new riders carving earlier. I had fun on the thing going slow, but as soon as my speed and momentum (200 lbs) picked up, the nose essentially folded over during a hard carve... on a green run.
I was able to carve faster on a shorter, stiffer board.
Obviously everyone wants a compromise, but if you gave me a 163 soft ass BX board, and a stiff 156 twin, I would choose the stiff shorter one.
After all... whats the point of having a big one if it's always soft and flimsy?