If you build a massive transportation infrastructure capable of hauling people & cargo for numerous several month journeys, you've pretty much adapted humanity to live in the space between worlds. At that point, why limit yourself to just Mars? As much as I'd love to visit, climb the mountains, rappel the canyons, and explore the lava tubes, there's a lot more solar system to see. Colonizing Mars adds one element of redundancy, but numerous self-sufficient space colonies, living off sunlight and the rich (and accessible) resources of asteroids would be far more robust - and interesting.