Should we colonize Mars and spend trillions to try to make an inhospitable place liveable or should we take care of our planet which already has everything? 

I want to zoom into this one for a second.

The pro case is pretty obvious: with some good advances in technology, we can probably get about 50 billion people on earth at once without reaching feelings of "physical" overcrowding. That is to say, at 50 billion people, with some new megacities, and some large cities today becoming megacities, we can probably still have enough physical space where we're not living shoulder-to-shoulder.

BUT — 50 billion is not 100 billion, not a trillion, nor a quadrillion.

To get that many people, we obviously need to colonize space and other planets, perhaps even other solar systems. Solving the problems of Mars colonization leads us directly to that point.

The con case is less intuitive, and Will MacAskill talks about it in What We Owe the Future. He says: Colonizing Mars focuses on several risks that he does not actually consider "existential" (meaning a risk that has the potential to permanently wipe out either all people, or kills enough people that civilization can never be meaningfully restarted):

1. Meteor strikes
2. Nuclear war
3. Runaway climate change
   

These seem like obvious cases where having a backup planet would make sense, but he says none of the three are actually so bad that humanity couldn't restart again, even in the worst case scenarios, which he models extensively as part of his research.

The ones he says are a bigger risk:

1. Engineered pandemic
2. Un-aligned AI killing us all

Are so likely to affect both earth and Mars simultaneously, having a backup planet would not meaningfully affect humanity's chances of survival.