I have both as well, the mexican kings are smaller as adults, but the black milks do seem to take a while to grow up. My milks are only a year old now and while they've gotten much darker, they still have a good amount of red on them. I've also heard that you can usually still see the bands in bright light when they do finally turn black (like two different shades of black) On the other hand, my black kings are solid black right out of the egg. Also the temperament on the kings seems to be a lot more variable. I've got some that are very tame and have never even tried to bite in the nine years I've had them, on the other hand I've got two that try to bite me every time I pick them up. The milks on the other hand (I've only got three) have never even struck at me.