I have about 60 milks and 10 kings. Most all of my milks are North American milks. As a general rule the captive bred milks are very manageable and do not musk or bite when held as adults. Some are flighy as babies but calm down. The WC milks I have don't bite or musk either but some are jumpy constantly slithering in your hands. Some WC are as calm as captive bred.
Of the kings I have most of the captive bred animals are laid back. But I do have adult captive bred easterns and speckled kings that musk every time I pick them up.
The sinaloans I work with are calm from the day they hatch. Same with the grey bands and Mexican milks I have.
So in my experience its difficult to say a king or a milk is more calm. It depends on a lot of factors including the sub species for milks and whether they are WC or captive bred. Some individual snakes just don't calm down even though their subspecies is generally though to be laid back like the eastern king example I mentioned.
I agree with others. You have a better chance at a calm milk if you get a Sinaloan, Mexican, black or Nelson's milk from a reputable breeder. These are often even calm as hatchlings.
Dave
DNS Reptiles