King Snakes are kinda dorky and.... hate to say... unintelligent. That's why so many people love em though, and THEY WILL NEVER SKIP A MEAL. Lol! Unless something is terribly wrong, like a sewed up mouth ect xD
Milk Snakes are relatively similar, more intelligent and more interesting natural colors. They have less color morphs than King Snakes but are arguably better looking with the banding and tri colors they can have. They can be a little hard to start as babies I hear but that isn't too common especially with CB.
Both are great choices, it's probably going to come down your preference of natural color vs mutations, and how many times you want a snake to attempt to swallow your fingers (a lot of Kings do this..)
There's TONS I didn't cover but I think those are the main things I'd address to anybody interested, such as yourself.
Good luck!
And, here's my Striped California King Snake just to give you an idea. I don't have any milks to share though unfortunately
Good luck on your decision!
PS - I also keep Boa Constrictors, a Macklott's/Savuensis hybrid and a Northern Pine snake and have had plenty others, if you ever want to chat ill be here! I can go on about reptiles all day..