Just a tip from full size racing.

A stiffer chassis will work better if it is set-up properly given a consistent racing surface. The set-up "window" where the car works is smaller because suspension changes have more affect on the car.

A more flexible chassis will often work better when the track is slick or inconsistent because spring and shock changes have less affect. The set-up window is larger but the chassis won't ever be "perfect". Sometimes this can make the lap times faster.