If these are the only two options (which I do not think they should be) then I would have to go for "exploiter." Columbus was clearly not (in my mind) mainly focused on exposing people he found to Christianity.
In my opinion, Columbus's main goals were glory, power, and wealth (three related things). He did not go specifically to exploit the people he found -- the went to make himself rich and important. But if I have to pick between him being an exploiter and a missionary, I would have to say that trying to get rich and famous is more like exploiting than it is like being a missionary.