I've been to the park about half a dozen times. It's an absolute blast. I am in Waterloo quite a bit to hang out with the in-laws. We end up rolling over to the park. They also have Dole whip (which is incredible).
If you are on the fence, go. It's a really nice park with incredible theming. I live in Ames and prefer Lost Island (than whatever Adventureland is).
As far as Worlds of Fun goes, it has way more coasters. If coasters are your thing, Lost Island is still growing and will be expanding in that area. It's a young park, and the family that owns it knows how to develop a theme (which will lead to better themed coasters).
Too long didn't read: theme park enthusiasts give Lost Island rave reviews. Go there. It's a blast.
I mean this sincerely… do people care about the theme of parks? I mean unless it’s Harry Potter or Disney or something I honestly don’t care.