Mars would be an ideal terraforming candidate; I don’t see any reason for human colonization until it’s a shirtsleeve environment. It also is within reach. Travel for months — one way — to take up living in a cave? Maybe preppers would be up for that. Nitrogen and oxygen in frozen form are probably available in the outer solar system, and engineering their packaging (in mineral foam ‘bubbles’) and delivery to the Martian surface (boom! boom! boom!) would give the planet an atmosphere within a reasonable timeframe, help develop the techniques and technologies for exploitation of space debris and deep space travel, and help with planetary defense against random large impacts.
Some day...
There’s nothing wrong with living in a cave - look at the inhabitants of Coober Pedy. Developing directional drilling, sealing, and subterranean living also will protect people from extinction events on the surface.
I disagree. The advantages of earlier colonization are considerable in terms of the level of interest that would be maintained, not to mention getting started while there is still interest and before something goes wrong here. Martians would care a whole lot more about solving the terraforming problems than we do back here. They would also be far less vulnerable to political whims and fads than we are. It's easy for our morons to whine that we should feed our hungry before we provide any funds for Mars, even though we can never feed all the hungry because of the choices they make. It's almost unimaginable that Martian colonists would take the same attitude.
I'd like to go to Mars as soon as we are ready, with people staying there and building a self-sustaining civilization. If we do it with people over 45 or 50 years old, so that they already have kids grown and out of the house, they can build without worrying as much about whether their children are in danger, and that might be a compromise from true colonization. However, I like the idea of building for my children. If we had a mission in two years, I'd be training, studying, and getting in shape to qualify and encouraging perhaps half of my children to come along.