As far as I understand, (correct me if I'm wrong) one of the reasons for wanting a colony in Mars is for the possibility in the future of having a space port where rockets could leave the planet cheaply, considering the lower gravity. Now this reason doesnt seem to have any advantage over the Moon, which is closer and it has less gravity. Which are the reasons for wanting to have a colony in Mars first than in the moon? Is it because it has more water? Is it because it shows more friendly features for human life even though life outdoors right now it's impossible? Is it because we want to see if there was or even there is some primitive life form there? What is it?
While I don't agree that we should have a Mars base before a Moon base, there's several compelling arguments you can make. I'm going to use Zubrin's answer on "why Mars?" In which he explains that "Mars is where the science is, Mars is where the challenge is, and Mars is where the future is".
Scientifically, we know more about the Moon than we do about Mars. Mars is also a more interesting target for science as it's highly likely that there was life on Mars at some point and it's possible that there still is. If life were discovered on Mars, it would be one of the greatest scientific discoveries in human history and definitely be a solid reason for a permanent base.
Secondly, Mars is where the challenge is or where the prestige is. You need to remember that the Apollo program wasn't about doing science or landing people on the moon, it was about convincing the world that the USA is better than the Soviet Union. Having a Mars base or being the first nation to land people on Mars would be a great achievement for any nation, far greater than landing people on the moon again.
Finally, Mars is a prime target for future human colonization and expansion. The moon can't really be terraformed whereas Mars could (in theory) be terraformed to sustain human life on the surface without a pressure suit (but still an air supply) within a human lifetime. Also, Mars could act as a fuel deposit or manufacturing base with easy access to the asteroid belt or other outer system bodies.
Well, Mars is more Earth-like than the Moon of course. It has more than twice the Moon's surface gravity, an atmosphere (with some parts of it having a pressure at or more than the triple point of water which means liquid water can exist outside because Mars barely but still is in the habitable zone), weather, natural satellites, is more than half the size of the Earth and it's a rocky planet. If you wanna travel further beyond to the outer planets and their moons, you can use a spaceport on Mars inbetween. Nonetheless, we'll obviously have to and should establish an outpost on the Moon first. The Moon in turn has other resources. And while both the Moon and Mars have no magnetospheres, both have local magnetic fields. SpaceX probably will fly its circumlunar tourist mission (DearMoon project) before sending people to the red planet.