This whole debate...
... is predicated on the axiom that there is some special reason why our race should survive. Evolution/natural selection inspires life-forms which seek to stay alive but by avoiding predators and so on, not planning centuries ahead. Perhaps the blind assumption that we must/should spread from Earth is part of the same thing?
In my opinion, any impact other than one which would literally destroy the planet would not wipe out humankind or life in general, and would leave behind more people than we could set up on a colony. We're not dinosaurs, reliant on a certain climate or delicate environment, but tool-users able to survive a wide variety of conditions.
Putting the time/money into building a framework - libraries, tools, etc - that would allow a remnant of the population to survive the harshest conditions following a major event seems better. For the cost of a program to build a self-sustaining colony on Mars, we could surely build 100 equivalent kits which could be used to re-colonise Earth!