While corns are a single species, kings and milks are both groups of snakes that have some real diversity - not as easy to make generalizations about them. Kingsnakes can vary from bold, curious and outgoing guys like california kings - that are more likely to bite and musk, but spend a lot of time cruising the viv and are very entertaining - to the very mild temperment and shyer nature of the greybands or variable kings.
Milks on the whole tend to be shyer than corns - they spend a lot of time hiding and they are more nervous as youngsters, though they can grow out of that and be very tame as adults. Again - huge variation in type depending on species - many North American milks mature at under 3 feet and some central and south american species get to 6 feet.
Temps are similar to corns, though some milks - like black milksnakes need lower temperatures. Humidity requirements really depend on the species - whether desert or coastal type animals.
Both kings and milks in all their variety are great captives - there is no forum comparable to this that I am familiar with.
best of luck in your decision - if you have some specific species you are considering, maybe post those options and you can get more information here,
mary v.