I always wondered about this comparison, I grew up where the red clay met the sandhills and I noted that very few have the opportunity to literally build their house on a rock. There were thousands built on sand that stood for decade after decade with almost no change but those built on the red clay sometimes settled and shifted, the sand seemed to be the more stable of the two. I guess it depends on what kind of sand it is and how dry or wet. We had a lot of dirt roads and we drove the sand roads in wet weather and the clay roads in dry weather. When it was really dry the sand roads could become impassable, same thing with the clay when it was wet.
Having grown up in Southern California (in a town where EVERYTHING was built on sand), I can tell you that sand is perfect - until you have an earthquake. Then what we call liquefaction occurs: so much kinetic energy is present in the sand that it no longer acts as a solid, but as a liquid. Of course, the flood problem mentioned in the parable is true, too, the two times a year it rains. "...and great was the fall of it."