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.