The number one reason the Moon is the most ideal place to create a permanent settlement in space is its proximity to Earth. The Moon is only a few days away with 1960's spaceships. This allows the Moon to have a very flexible trade route and exchange with Earth. If there is a problem help can arrive in days. If a piece of equipment is needed, its only a phone call away. Mars, on the other hand, will take months to travel to, and currently requires relatively precise timing in order to make the trip as efficient as possible. If you miss a particular launch window one would have to wait several months before another would arrive. Plus its distance creates a time dilation in transmissions that makes that part of the process slow and inconvenient.
But the proximity to Earth doesn't just aide travel time. It also decreases the risk to the people that work on the Moon. Though the Moon has no atmosphere and is barren rock exposed to space, it is still semi-protected by the Earth's magnetic field. This field decreases the overall radiation that hits the Moon's surface, as well as the ships traveling to and from it. This protective force field disappears as a ship travels to Mars. Though radiation is still present in heightened quantities on the Moon, one will sustain far less exposure than during a six month trip to Mars.
|A Lunar Spaceport|
One the most valuable parts of the Moon is its physical resources. The regolith of the Moon can be turned into any number of construction materials. Concrete blocks, sintered beams, rail tracks, these are all items that can be made form the lunar dust. The Moon also has deposits of water. This allows the colony to support its inhabitants without constant resupply. Water allows for farming, showers, and the creation of rocket fuel. (a very useful thing in space) Any base built on the Moon, with sufficient power emplacements, can continually grow and manufacture from the resources available. And the base is able to get from the Earth whatever it can't make, since the Moon is close enough to have an Amazon account. The resources and the proximity of the Moon allow a lunar colony to not only sustain itself but also create a surplus that can be traded to other space-farers. Options such as space stations have no ability to grow past what is provided.
|Lunar Habitat Made from Regolith|
Any early lunar colony would also be a target for funding from any number of scientific fields. Astronomy, physics, biology. A colony with excess facilities would be able to lease out space to to astronomers looking for an unobstructed view of the stars and others like them. And since the Moon lends its materials to creating excess space, this option would be a low cost, high return proposition for an established colony. Mars doesn't have the view and space stations don't have the space.
Lastly, the Moon has something that can be utilized on Earth, this is a necessary characteristic for any early space economy. Just like the missions to the Americas by the Hudson Bay Company, they didn't pay for themselves with exploration, but with exploitation, the same idea must be adopted here. The Moon has minerals such as Helium-3. This isotope is an ideal fuel for fusion, once the process is perfected, but is practically non-existent on Earth. The Moon is full of the stuff, due to the solar radiation that hits its surface. This material and others like it would be the tradable good that can be brought back from the Moon and sold on the Earth. As far as we know, Mars has no unique or valuable resources to draw from with a potential financial return.
Overall, the Moon is just a more reliable place to create something that can actually have an economic presence. Its resources allow for surplus creation and tradable products. Its proximity to Earth allows for easy and quick transportation. Its gravity and lack of atmosphere allow for easy interaction with other facets of the space community. And its environment allows for recreational value as well as reduced health risks. Space Stations, Asteroids, and Mars do not have the combination of qualities required to serve as the first true human settlement focused on economic return. Though all will be needed and are necessary steps in the conquest of space. Anyone working to create the ideal spaceport, to be the center of the Space Economy, should set-up shop on the Moon.