Why bother terraforming planets, or settling them at all? With current tech we could capture an asteroid, throw it into an Aldrin Cycler orbit, and hollow it out into an O'Neil Cylinder. Boom, instant colony which can hold hundreds of millions of people and naturally cycles between near Earth and near Mars every couple years. Repeat as necessary.
Space
Cover author: Michał Kałużny http://astrofotografia.pl/
I definitely think this is the cooler way to go. You could even put engines on them so that they could migrate around (slowly; I'm envisioning engines that modify their orbits, not allow for free motion). We could have space stations orbiting Mars and the Moon coordinating drones below for research and asteroid habs that can visit these stations for transfers.
But as the article/book points out, there are still a ton of questions we need to answer before that is possible.
Yeah, I think that is a very slept-on concept. There are MANY asteroids in our system which could serve the purpose. We actually have enough room in this system for nearly infinite humans if it's done that way.
So you want to have the Side colonies from Gundam?