As I've stated before Mars offers a credible source of Fuel whereas the Moon for the foreseeable future doesn't. This alone should include Mars in the list of things to do first.

As to selling places...Park isn't interested in Humans to the Moon either so if Spudis had been the person he debated. I'm sure humans to the Moon would get short shrift with him as well.

We can learn much from Mars about asteroid impacts in an atmosphere, and the question of life is important for all kinds of reasons and Zubrin is right. We can't build a robot even remotely controlled that in a reasonable amount of time could go to the local grocery and pick up a dozen perfect apples let alone find fossils on Mars. BTW, I don't buy the national security argument for the Moon or the "high" ground argument either. We can't guard a few thousand miles of earthly border adequately to stop "invasion" so the Moon is open to anyone with the means to reach it. Any defense of space would probably result in a field of debris we would be incapable of breaching thereafter.