I'm guessing that the difference comes from the depths of the fermenters. Modern cylindroconicals can have significant depth, while the straight side walls of a square fermenter make it structurally more difficult to make them deep. The 1000 bbl cylindroconical fermenters I saw at Sweetwater in Atlanta were probably over 40 ft tall.So it's the depth not the shape. At the homebrew level I am guessing it wouldn't matter.
A 6 gallon round bucket vs. a 6 gallon square bucket would make no difference?