Remember, though, that individuals will always exist which defy group norms.
In general, Mexican Black and Cal Kings have in comnmon that they are both about the same size and have voracious feeding habits. Each also tends to like snakes as food, maybe even more so than other subspecies. Feed generalists, they are easy to feed, as opposed to Alternas and other species which may not prefer rodents as prey.
How they differ is that MBKs are usually even more mellow as adults. The head is more wedge-shaped and wider at the jowels. Cal Kings have more narrow, rectangular heads. MBKs also tend to be slightly more stout than Cal Kings.
Both are exceptional species for "pets" (animal companions), with the Cal King usually slightly more animated & active. Cal Kings only tend to by "flighty" when juveniles. As adults, they tend to be quite mellow and handle well.
While all snakes can be quite mellow if handled well, Mexican Blacks just seem to win the prize for mellow. If you want a more active snake, they may not be as suitable.