As BDL stated both are very low in gluten but, what wasn't stated is that both are very high in starch and starch is a desired quality as far as thickeners are concerned. If you want to make a roux or thicken something, then cake/pastry flour is THE best thickener. Just think of cornstarch.
BTW there is an inverse relationship between gluten and starch. High gluten = low starch content whereas
Low gluten = elevated starch content. Using a high gluten flour (bread flour) to thicken something will give the liquid/sauce a gummy mouth feel.