I think the point was that however appealing Darke's arrangement may be, Holst invented the tune.
I take that back.
When I posted it I had never heard the Darke version and knew nothing about it except what I had read here, which suggested that it was an arrangement of Holst's tune. I have now heard it and read the Mudcat thread. Whether or not one can call it a completely different tune, and to what extent the similarities owe to the fact that they are settings of the same text are questions that will not be answered here.
But the Darke version is certainly something other than simply an arrangement of Holst's tune.
It certainly seems that people tend to prefer the version they knew first. I come from a background of classical music, not Christmas Carols. I prefer the Holst.