The choice is obvious.
In my opinion, after a certain point, will become an economic necessity; i.e. a lack of resources on earth & cheaper manufacturing costs in space will drive humanity spacebound.
If we have a viable self-sufficient colony outside of Earth (even on the moon or in space) humanity has a better chance of survival if we manage to mess up our planet too badly to fix.
I still can't think of what sequence of disasters would lead to Earth being less habitable than Mars. Earth still supported life the day after the meteor hit. To bring Earth down to Mars level something would have to remove all the atmosphere, remove all the mineral resources, remove the oceans, and screw up gravity and the diurnal cycle.