I asked Pappy about this a lot. It was important, too, in that he was always using a khuk to lop the head off a snake on his property. He had a hard time telling me which way to go. First, there's a lot of individual variances with any two khuks, so a real light 20" may be faster than a heavy 18" In general though, the 18" is going to be quick, and the 20" a little slower. The late Pappy wanted the extra two inches when he was dealing with poisonious snakes.
munk