King's Island is a major theme park, along the lines of a Disney, Universal, Busch Gardens, Six-Flags, Great America, etc. type of park. It's similar to it's sister park, "Kings Dominion" in Virginia, if you've ever been there.
Holiday World is a nice park but considerably smaller. It's located in Santa Claus, Indiana. I'd put it more along the lines of a "Dolly World" type park (Pigeon Forge, Tennessee) or maybe a "Silver Dollar City" type park (Branson, Missouri), or "Worlds of Fun" (Kansas City), but smaller than any of those three.
Both King's Island and Holiday World have associated Water Park areas as part of the theme park (no separate admission required), but King's Island is the larger one.
If you won't have a full day to spend, then Holiday World is cheaper and you can do the whole park fairly quickly. The roller coasters are nice but older and shorter. Nothing like King's Island which has the #1 rated wooden roller coaster in the country ("The Beast").
As others have mentioned, Cedar Point is also a great park, but it's not clear where you'll be coming from, so Cedar Point could be out of your way.
One thing unique about Holiday World is they have unlimited free beverages. (soft drinks, lemonade, ice tea, coffee) There are stations around the park. Just go in, get a paper glass and self-serve. In the water park area, they have free self-serve sunscreen lotion.
I assume you've gone on-line and checked out both park's websites. (
www.visitkingsisland.com and
www.holidayworld.com)
Hope this helps