Here is the comparison between the HD and SMC 21 LTD. The focus was poor on some of those shots, sorry. The K20D is at its limit handheld in these situations.
HD 21
SMC 21
HD 21
SMC 21
What does it prove? Not much I'm afraid. The SMC limited are already amazing in their own right. There seems to be slightly less CA in the out of focus areas with the HD, but really I might be imagining it. As I wrote before, the build is the same, I prefer the lettering of the SMC (the font on the new one looks slightly cheap, to tell the truth). The red ring has the same look as the green one (shiny, metallic-looking, etc).
It may be that with a 24 MP sensor, the difference becomes more important, and my humble opinion si that this is the reason why Pentax released the new versions, in order to really take advantage of that sensor. It would make sense.