All three have essentially no atmosphere so that is not the issue; we would still have to wear spacesuits. At least the Moon and Mars are known to have sources of oxygen locked up in their minerals, and we can also extract oxygen to breath from the water, but we also need nitrogen and that element seems harder to find or extract.

Mercury is so close to the sun that any x-ray flares will not have their radiation 'lethalitl' diminished by the simple inverse-square law like they would from, say, Mars which is a factor of 4 times further from the sun than mercury so we would get a factor of 16 reduction in radiation levels from that alone. Every little bit helps, because radiation exposure is one of the most severe hazards the astronauts will have to deal with in the journey.

The simple issue is how much it costs to get a pound of mass from the Earth to each location, and by far the cheapest is the Moon so it will be 'colonized' first based on simple economics alone. On the other hand, we have already been there, so we cannot discount the 'novelty' factor so we may just decide to colonize Mars next. In fact, in 2004 President Bush proposed just such a goal for NASA to work towards.

Every plan I have seen says we will launch a series of cargo ships to Mars from Earth-orbit and not from the lunar surface. After these are in orbit and have deposited their cargo at a main location, robotic activities will start generating rocket fuel in an automated plant, and the supplies will be thoroughly checked out at the landing/colection site.

We will then send a crew of 6 or so astronauts to land, and collect the cargo to set up a base which they will use until the next Earth-Mars opposition 780 days later. So setting up a real working colony will be a necessity on Mars, but not as much of one for the Moon.

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