One point which is made in the four part article I posted is that magnetics play an important role.
The other point is that a feasibility study must may made to determing the magnetic fields size and the solar winds both planets. This has a lot to say about being able to live on Mars. What are the G forces on Earth and Mars? One point you made Bob about compass failure is that most compasses work off of the magnetic north and south poles. If there is a difference between Earth and Mars, this could through the calibration off. It may even affect communication relays on the space craft.