To the Moon for sure - for humans. Because the Moon is interesting in its own right. It's like ignoring Antarctica because someone landed there in the nineteenth century, so it is already "done". It's the closest place to us, and easiest to explore. If we can't explore our Moon, we have no chance of exploring Mars.
Also - why rush humans as quickly as possible to the place in the solar system most vulnerable to Earth microbes? We want to find native Mars life there, if it exists. It is easy to find life if you bring it with you yourself, but that would be the worst possible anticlimax of our search for life in the solar system. Especially if we found that there was some vulnerable form of early life on Mars, say, RNA based life which on Earth got made extinct by DNA life, until just before the humans got there. We must make sure this can't happen.
We can explore the Moon which is turning out to be much more interesting than most expected - it's also resource rich. It has some resources that Mars doesn't have indeed.
Then as we do so, we continue robotic exploration of the solar system, and find out what humans do best, and what robots do best, starting off with humans on the Moon. Then continue outwards in an open fashion, building on what we've learnt.
I've just written an article for Science20 trying to set out a "Case for Moon" as a positive vision for the future like Zubrin's "Case for Mars".
It can be an exciting future, exploring the caves on the Moon, the polar ice, searching for Earth meteorites from early Earth (about 200 kg per square kilometer of Earth meteorites there), even meteorites from early Venus and Mars. Ice at the poles but not just that, also CO2, CO, ammonia, etc. The Moon is actually rich in many metals such as aluminium, titanium, and as for iron, there's iron powder, about half a percent of the soil, which you can separate out with a magnet.
Later on we can explore Mars from orbit and its two moons, Phobos and Deimos. Meanwhile continue to explore Mars robotically, and also later, telerobotic exploration from orbit.
In this approach, science and planetary protection is central. Space settlement happens because you are there for a purpose. And as with the Antarctic bases - once we are there doing good science, with science as the motivation, then it would be an on going permanent first step into space.
Not just a "boot print" mission - once it is done, the public will wonder why you want to continue having clearly fulfilled your objective. And colonization - I think would just fizzle out, not being sustainable except with billions of dollars a year support from Earth which would sooner or later dry up.
But missions motivated by science just grow and grow, and engage the public - and what's more though, they can also have overwhelmingly positive outcomes too, especially if we make new discoveries about biology and evolution.
On the Moon we can have human missions where the humans are actually there to explore and make discoveries about the Moon, not just to study themselves in zero g and study how Earth organisms and such like behave in space conditions. It can be our first go at finding out how humans get on at that, and the relative rolese of humans and robots, close to hand, where you can get back to Earth within a couple of days in an emergency, also resupply them at any time witout needing to wait for special launch windows and journey times of months - by which time the emergency is probably over one way or the other.
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