I don't buy the arguments for why Mars is better than building space colonies. For example:
Mars is really not far away; a quick look at a map of the Solar System shows how close it really is.
Maybe compared to Pluto it's "not far away", but that's an irrelevant comparison! You might as well say the Andromeda galaxy is "not far away" if you look at a map of the universe. What matters is how far away it is compared to human length scales. How long will it take to get there, and how much energy will it take to do it? The closest Mars comes to Earth is about 56 million kilometers, and it only comes that close about once every two years. That's an incredibly long way! Even if you assume miraculous new propulsion technologies will allow us to travel several times faster than is currently possible, and do it with using ridiculous amounts of energy, it will still take months to get there.
Besides, that miraculous new propulsion technology would work just as well for other purposes, like asteroid mining. So if you want to assume travel to Mars can be made fast and inexpensive, you also have to assume asteroid mining will be fast and inexpensive. You can't use optimistic assumptions for one and pessimistic assumptions for the other! Furthermore, the requirements for asteroid mining are a lot lower than for Mars colonization. If it takes a few years for a robot to tow an asteroid into position, no one will care much. If you have to spend six months in a cramped ship en route to Mars, you will care very much.
Mars provides a land area equivalent to the land area of Earth, which is a huge platform on which to build. In free space you have to build the platform first, using resources that need to come from somewhere else. Until we have mining facilities on the Moon and asteroids, this place will be Earth, which is a much deeper gravity well.
That hardly seems like a big deal. In either case the colony needs to be fully enclosed. On Mars you only need to build the upper half of the enclosure, while in space you need to build all of it. Either way, the cost of building the enclosure is likely to be small compared to building everything inside it. And of course you would mine the materials from the moon or asteroids. Getting them from Earth would make no sense at all, so why even bring that up? It's a straw man.
It's true that we don't know if living in 38% gravity long term is healthful, but since we know living in microgravity is certainly not fatal, it's reasonable to imagine that people will adapt.
That's a huge assumption, and completely unjustified. Six months in microgravity doesn't kill you, but it does cause all sorts of health problems, from bone loss to vision problems to (as we just heard today) skin becoming thinner. Is 38% Earth gravity enough for people to be healthy long term? At present, we just don't know. All claims to the contrary are wishful thinking. And if the answer turns out to be no, then all plans for Mars colonization are dead on arrival. So maybe we should try to find out before spending too much money on those plans?
Here are some other serious problems with Mars:
It has no magnetic field, which means no protection from cosmic rays. As far as I can tell, there is no possibility of ever changing this, which means no possibility of people ever living out in the open there. Even if you terraformed an Earth-like atmosphere, the radiation at the surface would still be too high to live there safely. So people on Mars will always have to spend their lives in sealed habitats behind thick shielding. All claims to the contrary are simply unrealistic.
There is very little energy available on Mars. The only mostly reliable source of energy is solar, and not so much even of that. Sunlight on Mars is only about half as intense as on Earth... except during dust storms (which can last for weeks), when it goes down to much less. And it's only as high as that because Mars has no atmosphere to speak of. Terraform an Earth-like atmosphere, and it will go down by half again.
In contrast, put a solar panel on a space colony near Earth, and it will produce 6x as much energy as on Earth. 2x because it's above the atmosphere, and another 3x because it can be pointed directly at the sun 24 hours a day.