I really hate that smart comment. Two big differences are that with overpopulation on Mars, (1) we would be able to address it before it happened or while it was happening, on a time-scale of decades, and (2) even if there was a total disaster and Mars became permanently unlovable and a lot of people died, we'd still have Earth. Whereas (for those concerned about runaway AI) the time-scale could be minutes and the stakes are the end of known intelligent life in the universe.