A corn snake is actually a type of rat snake. King snakes include sub species like the milk snake. The differences between them are not really that apparent to the average hobbyist. Various species between the two groups really just vary in size and colors mostly. They both have very similar bodies and scale types. They both will eat mice. The care will be identical for the most part. This is more opinion than fact, but my milk snakes have always been more night creatures, and most of the corn snakes I've had or seen don't really care and come out day or night. Some species get bigger than others in either family. You would want to think about the maximum size you might want and that could narrow down the species you might want. ALso, the availability in your area or whatever. All in all, corn and milk snakes are fairly easy to care for hardy snakes. Just read up on some care sheets and give them the correct amount of everything they need.