I didn't know both claimed to be the tallest. I saw this over at Wikipedia: Kings Island's tower is the tallest in the world. While Kings Dominion's tower isn't taller, the brakes start farther down, therefore providing a longer drop than Kings Island's.
Let's get into a "tallest WS" competition with the other parks, we could compare how far above sea level each city a WS is in and claim the tallest based on that.
I don't think that is right because I am sure they prob. are counting the brakes as part of the drop.