I am having a very hard time understanding any climate scenario that would make Mars more habitable than Earth. Why should we spend a dollar getting to Mars?
If the worst comes to pass and the species needs to throw a hail mary, I have to think our chances of success would be improved (even if slightly) if we were to have these sorts of details ironed-out in advance.
Suppose we started to really use Fusion power not just at our current energy needs, but enough to directly warm the planet. There are hard limits on just how much heat per m^2 you can dump before you start to cook the planet.
Such a civilization may prefer low atmospheric pressure and a planet further from the sun simply for cooling.