Although there is no hard and fast rule, my own understanding of chassis flex is:
thinner chassis: useful for outdoor,lower grip tracks.
thicker chassis: good for indoor,high grip tracks, especially with foams. this is because foams probably give a higher mechanical "bite" over rubber tyres.
The stiffer chassis gives the car more of a feeling like when you lower the roll centre, it transitions the car left/right quicker.
hth
