Hog. They went to the 3-piece maple around '74/'75. It's one of the reason why, when I am looking for Norlins, I always look for the late '70s ones. 1977-1979 Les Pauls are my favorite guitars ever made.
I'm sure there is no science to it. Or I guess I should say I would be shocked if there is. All I can say is that, anectodally, the maple-necked Les Pauls I have owned have been the best sounding, best sustaining and most stable of all. They have a note definition and separation that I have never heard from a hog neck and that makes them work so much better with high gain, IMO. They seem to resonate differently from the mahogany necks, in a snappy alive way like a Fender, and it seems like the hardness of the wood (again my perception) keeps the strings vibrating longer. And they almost never need the truss rod adjusted (I do my hog necks once or twice a year, maple once a decade), come out of the case in tune every time, hold tune every time, just utterly stable.