[blkhawk]

Mars might not be worth the effort if you have good asteroid mining - you could say build a O'Neil type cylinders inside an asteroid from local resources and spin them up. Some of the Tech you need for mining might transfer directly into tech or building structures like this. For instance refining metals in zero-G via rotating smelters to separate materials might serve as system to spin up and control the cylinder rotation.

You might even get most of the stuff aside from metals as byproducts of the metal refining process (water, oxygen(from oxids) and carbon mostly). Nitrogen and Phosphor might need importing.

Most of the leftover asteroid serves as Protection against small collisions.

[elihu]

If water/oxygen/carbon are readily available, I guess that would go a long ways towards making asteroids self-sufficient. On the other hand, if they're available but one can only extract a small amount at a time as a byproduct of mining, then Mars might still be pretty attractive where those things are available in inexhaustible quantities with low effort.

I'm assuming that huge amounts of methane (or other suitable fuel) and oxygen will be wanted for propellant, though maybe ion propulsion makes that less essential.