Moon or Mars? It isn't a mutually exclusive choice but we'd be idiots to ignore the ideal staging post.
NASA engineer, Wingo, makes a detailed, costed argument that the current best-of-breed technology should be directed to the Moon. Specifically, the Saturn program [wikipedia.org] should be continued in preference to SLS [wikipedia.org]. The reason is quite simple. With advances in manufacturing, materials and guidance systems, a known quantity with known corner cases can be made safer and cheaper. (It would also avoid launchpad upgrades [nasa.gov] and other superfluous costs.)
As a matter of international co-operation, this could be augmented with Russian technology and suchlike. Yes, redundant airlocks or airlock adaptors may be required. However, does it really matter if a substantial structure requires seven payloads or eight payloads? From our current position eight is cheaper and more certain even if seven would be better in the long-term.
What would this structure be? A waystation in high Earth orbit for fueling and crew transfers. Fueling of what? Initially, craft to bootstrap a permanent base on the Moon with solar and nuclear power. Fueling is also needed until there is sufficient infrastructure on the Moon to produce fuel locally. Even then, fuel is required in high Earth orbit for emergencies. Overall, this is a plan to go from zero presence to an economic break-even point and beyond.
A mineral mining expedition to the Moon has an estimated ROI [wikipedia.org] of 22 years. More worryingly, the total cost is dwarfed by student loans, mortgage fraud and bank bail-outs - and that's just counting US figures. That's the most damning part. If we never get off Earth it will be due to the soul-sucking 1%ers and the legions of [dailymail.co.uk] B-Ark [wikipedia.org] space-cadets. On that basis, we deserve to not get anywhere.
Admittedly, figures for mineral mining assume that a glut in the market won't cause a price crash. There is a certain irony that a mining expedition to the Moon may never be economically feasible if it makes resources too plentiful. But seriously, that is a risk worth taking because it provides opportunity to move the majority of heavy industry outside of the biosphere. Even ignoring this, it would be possible to drop titanium airships into the atmosphere with a cargo of tritium from the Moon's South Pole. Or lithium. Or neodymium. Do you think there's enough lithium or neodymium for everyone to have an electric car? There is if we mine the Moon. (Or maybe that's why we don't go? Would we use the resources sensibly prior to mass population reduction and careful management of MTE [soylentnews.org]?)
The typesetting is a bit dodgy but the message is clear. Until transport to the Moon becomes routine, human missions further afield are a work of speculative fiction. Actually, there comes a point when sending robotic probes further into the solar system becomes cheaper when sent from the Moon. And that's the point where we should seriously consider further expansion. Not before.