About the only thing that'd make it harder to survive on Earth than on Mars is a planetoid-sized impactor. For anything else a few hardened, isolated bunkers would be a hell of a lot cheaper than colonizing Mars, at least as effective at improving human survivability, and we could start building them today.

Of course I'm rooting for the Martian stuff because it's cool and may be accidentally useful (SpaceX already has been, really) and beats the fuck out of finding new ways to sell crap to people and the other garbage lots of other prominent companies do, but the goal isn't valuable per se, in my opinion.