I'm with replaybob on this... Mars does not really seem like a good target world for human colonization. In addition to what he says, there is also the great difficulty on landing there. Since the atmosphere is so thin, parachutes do not provide adequate braking, but since the gravity of Mars is fairly substantial (more than twice that of the moon) it takes a lot of propellant to slow down a large mass.
The world we should REALLY be targeting is Titan, largest moon of Saturn. Dense atmosphere (1.6 times Earth's!), low gravity, and all kinds of hydrocarbon resources (fuel, heating, and water for drinking and disassociating for oxygen). Easy to land there and lots of indigenous supplies. Only problem is its REAL cold. But you could inflate structures with water instead of air, and they would freeze into solid like rock.