not fleet pride.
shaft design is a function of design constraints and angles, double cardan is for when all the angle is at the tcase, and you're running within about 3Ëš of 0 angle (minimum 1Ëš) from pinion to shaft.
single joints are for when pinion angle to shaft and case angle to shaft match, in either direction (4Ëš down from case to shaft and 4Ëš up from shaft to pinion, or down and then down)