The heel counter and a more substantial backstay, and the bellows tongue, and the option of a rolled top. There may also be some differences internal to the build, but I'm not sure. But I do know that the BH feels much heavier and more substantial to me then the SD.
I had a pair that I wore 4 or 5 times a week for ten years. They wore like iron, and the leather, which was not much at first, took on a great patina. I tried on a pair of the new service boots and they felt much lighter, with the sort of flaws you have seen.
I'll just say that I have the lowered block heel and that I like bot the way they look and the way that feel. It's been said many times and many ways, but basically, Different strokes for different feet..