Mars is more habitable than Earth would be during an extinction event. After the event the wiped out Earth would certainly be more habitable, but if there aren't any humans available to repopulate...
There is a broad range of possible human extinction events, e.g. asteroid impact, large-scale volcanism, viral epidemic, nuclear annihilation, biological warfare, ecological collapse, and autonomous robots [1].
Humans actively monitor and defend against each threat. In the U.S., NASA is committed to detecting asteroids, and recently successfully altered the course of one [2].
I believe we are most powerful to overcome extinction events as a united species on the planet we’ve evolved to live on over billions of years.
