Almost everything about the plan to go to Mars, especially about creating a "backup population" is uncertain. Compare that to the current, immediate suffering and lost potential that is caused by poverty and disease.
An catastrophic asteroid impact is a low daily probability, with an impact of up to one (one being complete wipeout of the human race).
I think it's a tossup which yields better happiness or productivity for humanity to address in the short run. In the extreme long run, of course, if you don't solve the second, there's little point solving the first issue.
The problem is a lack of resources - that resource being some collective focus of humankind, as there's no real physical resource limit preventing us from pursuing both at the moment.
Mars is a vanity project however he chooses to dress it up. An interesting vanity project sure but a vanity project all the same.
And personally I have no issue with that, I just wish he'd be honest. We didn't go to the moon the first time for any good reason, we did it because it was there. Governments can't afford to pay for the "because they're there" projects any more but if Musk, Page and co can then great but don't pretend it's for the good of humanity - "because it's there" is all the justification you need for something that amazing.
They're both great goals and I'm glad we live in a time when going to mars is something we can propose as a serious point of discussion, instead of being an unrealistic dream.
"Almost everything about the plan to help Polio victims, especially about creating a "polio vaccine" is uncertain. Compare that to the current, immediate suffering and lost potential that is caused by poverty and disease."
See how this works?
There are some very good reasons to pursue the colonization of Mars, mostly in the form of advancing science. A backup population is not one of these good reasons.
Colonizing Mars will be a force mulitpler as well, mostly through indirect technology benefit and ancillary space industries(asteroid mining). It probably won't impact the effort to cure polio but will probably increase our ability to cure other diseases.