At a fundamental level, space development is really about fundamental quantities: Energy, Mass, and Volume.
It is inescapably true that these resources are limited on the planet Earth. Expanding our purview, however, destroys this limitation, opening up myriad new possibilities.
Various arguments have been presented for why Mars is the greater objective than the Moon. But in the end, it's not really an either-or question. We can do both, we should do both, and we will do both.
Some key elements of the argument for Mars are:
- Sheer mass and real estate available
- Possibility of terraforming
- Atmosphere (protection from radiation and meteoroids)
- Day/night cycle and surface temperature