2nd moment of area, reference axis question
It depends on the problem you have to solve.
If you have a beam with a single force on the point of gravity of the profil and you want to keep it torsion free, take the I-beam. The beam is kept torsion free because of the geometrie of the beam. The shear of the left and right side of the upper/lower flange compensate eachother.
If you have a single force on the point of gravity of a C-beam you will find a resulting torsion over the entire beam. If you want to get rid of the torsion you have to move to force away from the point of gravity in direction of the x-axis so that you create a anti-moment on the beam which is exactly the same as the shear that is created by the upper and lower flange.