2nd moment of area, reference axis question
Zapboy's answer is correct, but in reply to Zapboy's question:
Vinzenma is right when he says that to not induce a torque, you would need to offset the load some distance from the center of gravity, called the shear center.
As for which is better, channels are often used because the one flat side often makes them very easy to connect to. With an I-beam, or Wide Flange shape, you often have to cut one or more flanges to make a clean connection. Naturally, a Wide Flange shape is more efficient in that they generally have a greater section modulus per pound of weight than channels do and they don't have the shear center problem.
As is usually the case with engineering, there's often more than one answer - it's just which one better fits your case...