Moon first, then Mars...
I have to laugh at the stupidity of people who think or say about a return to the Moon-- "Been there, done that". That'd be about like saying after Columbus's second voyage "BTDT" and so burn all the ships-- no need to return.
Exactly 12 people have walked on the moon. 24 have orbited it. That's it. 2 dozen people in all of human history. 6 places on the Moon have been explored by humans, with maybe that many more having been visited by robots (Surveyors, Lunakhods, the Chinese Rover last year...) That'd be about like parachuting 6 guys down in various spots across the US and saying it's been "explored". There's PLENTY of science left to do. If nothing else, we need to learn how to operate in the deep space environment, and on a hostile planetary surface. We need to do research into ISRU, or using deep space resources to create rocket fuel, water, oxygen, etc., all things that will be needed in bulk in any time of long-duration space exploration mission, long term habitat, etc. We need to learn about radiation shielding and long term low gravity operations and effects on human capabilities. We need to learn how to make a space suit that's not falling apart after TWO DAYS of surface operations... (The Apollo astronauts reported that their suits and seals were REALLY starting to show wear from the contamination of surface grit after only a matter of hours of actual surface operations-- which varied from mission to mission... Apollo 11 astronauts only were out on the surface a matter of hours, while by Apollo 17 they basically worked two days out on the lunar surface, and basically Gene Cernan said he felt his suit was "falling apart", that the seals were visibly worn and he worried that another day might see it fail entirely). These are questions that TENTATIVE ANSWERS are being researched here on Earth, but which CANNOT be FULLY TESTED on Earth-- our weathering processes create particulates completely unlike the lunar environment which has no wind or water, and it's quite likely that the particulate matter on Mars, although wind-blown like Earth, will be markedly different chemically then Earth dust and grit. We can GUESS, but we don't KNOW, because NASA doesn't even have the money to do a SAMPLE RETURN MISSION from Mars via robot, let alone anything more substantial.
As far as "living" on the Moon or Mars, I think that MAYBE, in 100 years or so, it MIGHT be possible, if technology continues to advance. Whether it will ever be practical or not or worth the effort, that's something else entirely. After all, Antarctica is a veritable paradise compared to the Moon or Mars, and nobody lives permanently there... we maintain a scientific outpost system, but there's no talk of "colonizing" Antarctica, despite the fact that materials and supplies, fuel, food, etc. are INFINITELY easier to deliver there in bulk (by regular ships or aircraft most of the year), oxygen is free and in infinite supply, water is easily obtainable by simply melting snow, etc. The Moon would be at least an order of magnitude harder to "colonize" than Antarctica, and Mars a magnitude harder than the Moon, if for no other reason than distance. If we EVER WANT to actually send people to live there, we need to answer some very basic questions first... We need to know if the lunar polar regions ACTUALLY contain water, and in what form, and if it's economically viable to extract, and how best to mine those resources and extract the water. Once we have water, it's easy enough to electrolyze it into hydrogen and oxygen via solar power (or nuclear power-- although the Moon gets equivalent solar energy levels on the surface as Earth (actually somewhat more since there's no atmosphere scattering the solar energy, no clouds reflecting it back into space before it reaches the surface, etc.) it is only available for the 2 week long day on the Moon-- and you need power for the 2 week long night. This will create big challenges for designing lunar power systems-- either you have to have TWICE the generating capacity in solar cells and then some sort of storage system for that power (batteries, or electrolyzing water into hydrogen and oxygen that you then store for use at night, running it through fuel cells to reconvert it into water and electricity, perhaps in a "closed loop" type system where the water is recaptured and fed back into the electrolyzers for use the next solar day, or to hydrogen/oxygen tanks for use as breathing oxygen or rocket or surface system propellants (such propellants could be used for long-distance rovers powered by fuel cells, mining equipment, etc). We have to understand how to build and maintain at least a basic self-sufficiency in energy, oxygen, and water for life support at a minimum before we can seriously consider long-term Mars operations.
Unless of course you want to do a Mars mission just for the sake of doing Mars... basically another 'flags and footprints' operation, with no lasting, sustainable presence possible. We could do an "Apollo" style Mars mission in a decade if we so chose to, and were willing to fund it properly and kick NASA in the pants about getting stuff done instead of playing around for years as they have with Orion and Ares/SLS. Sort of a "don't TALK about it, get it DONE!" attitude like NASA had in the 60's... (not that I see THAT happening any time soon, if ever!) We can shoot people out there in some sort of hab module for the year long ride out to Mars, land them, and then take off within 30 days for the return trip. You can't stay longer and get back to Earth, without waiting a year and half or so until the orbits realign (we have to be on the same side of the Sun to go or get back, basically). Heck even operations of 30 days on the Martian surface would be enormously difficult compared to the experience set we have from Apollo... It might be that a mission goes, lands on the surface for a matter of a few days, and then returns to Mars orbit and starts back for Earth...
Personally, I don't really see it happening... there's just not enough 'motivation' to pay for it and do it. Dreamers dream of it, visionaries proclaim it, but realistically, when it comes down to it, there's little motivation and less desire to actually fund it. Even now, with NASA loudly proclaiming they're "on the road to Mars" and all this with SLS and Orion, when one looks at the actual systems and costs, it's unsustainable, incomplete, and will require a FAR, FAR larger commitment in money and resources than has been allocated to it. Even by NASA boasting, they won't be ready to go to Mars for another 20 years, AT LEAST. When one considers how far robotics, computers, and ROV technology has come in the last 20 years, by the time we might THINK we're capable of going, with sufficient investment to pay for the hardware to do it, we should be able to land FAR, FAR superior robotic probes on Mars than ANYTHING we can envision today... I'm talking about robots that can walk or roll for miles, operating via their own on-board artificial intelligence, having received orders from Earth for their next objective, and deciding for themselves how to best get their, making decisions en-route as to hazard avoidance, obstacles, etc., and making scientific observations and recording anything of interest, and reporting its findings back to Earth for scientific analysis. If the robot sees anything of interest that it has highlighted for investigators on Earth, they can easily order the robot to backtrack and make further observations or scientific tests of the object or phenomena... And we could land a FLEET of these things on Mars, all over the planet, for FAR less than the cost and time wasted on even a MINIMAL human landing effort. These robots could operate on the surface of Mars for YEARS, even decades, and never need return to Earth. They don't need oxygen or water, or food or waste disposal. They can be engineered to operate in the environment autonomously without the need or possibility of repair (although programming them for self-repair is certainly a possibility in the future). They can be programmed to collect specimens and rendezvous with return vehicles from Earth, if we desire actual specimens to subject to more complete analysis here in Earth labs) which will return the samples autonomously.
Of course this side of NASA is being starved of funding and is languishing, largely BECAUSE it replaces the human and the need for "human exploration". If we can do the SAME THING robotically far more cost effectively, over a longer time period, and in more places, without risk to human life, and obtain more scientific knowledge and information in the process, then we SHOULD be doing it, and forget about putting "boots on the ground" there just for the sake of it... is it about "waving flags and photographing footprints" like Apollo (much as I love Apollo), or about achieving REAL scientific results and greater knowledge about a much broader region of our solar system than human exploration could ever think of achieving in the foreseeable future (or the next couple centuries at least). We've barely scratched the surface... our robots are currently at the "Ford Model T" stage, but they're not going to stay there long... look at military ROV capabilities, and you can see that developments are moving fast... in a couple decades what we can do NOW will look positively archaic... Remember Sojourner?? What a thrill that was-- a shoebox size rover on Mars... compare it to now. Now think 20-30 years from now...
There's plenty of places in the solar system where humans will most likely NEVER set foot... simply because it's impossible to set foot there and LIVE... The surface of Europa is such a place-- orbiting in one of Jupiter's MASSIVE radiation belts, landing on the surface would be about like camping out in a tent under a multi-megawatt nuclear reactor going full-power... You'd be bathed in lethal levels of radiation that would kill you in a matter of hours... Mars isn't anywhere near that bad, but if it were, we wouldn't have this massive divide between the "robotic people" and the "human exploration people". If human exploration was impossible, we'd throw everything we have into robotic exploration.
Later! OL JR