Double D shafts look too agricultural for my tastes, though they do have the advantages Walt and Bob mentioned. A slight advantage to the spined shafts is you have slightly more clocking ability for alignment of the column, steering wheel setup. Most of the quality spined shafts have 2 inches of spine at each end. I buy one as close to two inches longer than needed as possible and cut off the extra from each end which minimizes the amount of exposed spine sticking out from the joints (again a cosmetic issue, but details make the difference on hand built cars). Cutting off with a cutoff saw should give you a nice even cut, just grind a slight bevel on the cut ends, makes it slide into the joints more easily. Also plan to counter sink a set point where each of the set screws hits the shaft, gives a more secure set.