I'm not a mechanical engineer... but to me the new method of attaching the gear to the screw is an improvement. Using fewer parts and reducing the number of attachment points should reduce friction forces. And any force put on the "nut" while raising/lowering will cause it to wobble, introducing unnatural forces and eventually bending.
Call me paranoid, but another reason I would "prefer" a D is that it will have been manufactured more recently and thus spent less time in a warehouse or in the hands of a shipper. Why wouldn't I want one that has come right off the line, straight to a cargo plane and the dealer vs. one that's been where for the past 3 months?
That said, I'm accepting fate and keeping my B. At least it came with 14 props. I will be careful to operate the gear only on low friction surfaces and lubricate the screw per DJI maintenance recommendations. Cheers.