To effectively show a contrast between Mount Everest and Mauna Kea, you might consider using a word or phrase like "height" or "elevation." For example:
"While Mount Everest is the highest mountain in the world, Mauna Kea has a greater elevation when measured from its base on the ocean floor."
Here, "height" emphasizes the distinction in how we measure the two mountains' heights relative to sea level versus their full elevation.