This is so obvious to me I wonder why I beat my head against the wall trying to get others to see it while getting insulted in the process. The mars advantage can be summed up in one word: GROWTH.
O'Neill colonies? Take a good look at the ISS. How's its growth rate been? You're saying something a hundred times it's size or more will happen quickly? That they can cancel the rocket equation? (Not just from earth to LEO, but anywhere they go for resources.)
The moon? Missing essential elements. Does anybody have a clue to how that effects growth? Apparently not. Space and the moon require infrastructure that will be a long time coming even if they did focus on it which today they do not. Someday perhaps.
I claim mars can be bootstrapped. (The idiots that only know how to ridicule can leave now.) What this means is mars only needs two things to grow. A good start (as little as 10 tons of supplies per each colonist for the first dozen colonists, just one ton each thereafter. They can skimp on the thereafter, but shouldn't. Elon plans for a much greater ratio of supplies which is good to hear... can't wait to hear more in Sept.) and a steady inflow of people. The reason this works and results in growth not seen since the space age began is because of bootstrapping.
Martians will not need the imports that other destinations can not survive without.
At this point somebody will bring up computers (or if they're really stupid something the martians can make for themselves which happens much more often than you'd expect from otherwise intelligent people.) They can live on mars without computers as humans have for thousands of years, but they don't have any need to. New colonists will bring all the things martians can't make for themselves with them. Any ticket should include some personal property mass which becomes a valuable asset for new arrivals. They should avoid bringing things with them the martians can provide.
What this means is the fastest growth possible. Mars can handle and needs as many people as will go. Earlier colonists will be raising the level of habitation before the new colonists arrive using easily collected and abundant resources. Resources they literally pull out of thin air and dust. For the one time cost of getting them there, colonists will produce a lifetime of wealth. Both Zubrin and Musk believe they can get the travel costs down to where a middle income person could finance their own ticket. I hope they're right.
Iron for machinery is so abundant and easy to get they won't even bother to refer to an iron age.
Carbon and hydrogen, required for plastics and food will be pulled out of their surroundings with no need to prospect. Water is so abundant they'd have to make an effort not to find it.
Chemistry is not an unknown and the entire earth is available to assist. Mining and construction are also not unknowns. The only unknowns are rates and evidence suggest some of those rates (like plant growth in a carbon rich environment) will even be greater on mars than on earth. Side question: If plants evolved on earth why are they carbon deprived? Just wondering.