You do have it reversed. Mauna Kea is extinct (it better be, as it has several very expensive telescopes on its summit). Mauna Loa is still considered an active volcano; it last erupted in 1984. It's sorta kinda getting overdue for its next one.
Whoops, it was Kilauea that I was thinking of. I was on top of Mauna Kea just a few years ago. A beautiful place. Nary a bubbling cauldron in sight. :)
We also hiked around the lava fields at the ocean’s edge and got to play chicken with lava floes. Dang, that stuff’s hot!