Which one is stronger,going to last longer and is one more expensive then the other(material itself)?

The answer is... that depends. Pound for pound in equivalent material, I-beam is stronger. Can't say as I've ever seen an I-beam steel trailer though.

C-channels are the bomb. The dont flex and ride much smoother than I beam. The weld helps. Go with 8inch cross members on C-channel versus 6 and it is that much stronger.

Talking about aluminum

Chris said it: It depends.

Given an I-Beam and a C-beam having equivalent overall dimensions and wall thicknesses (same cross-sectional areas), they will be equally "strong" in tension, compression, shear and even short-beam bending when the bending is applied relative to each respective horizontal neutral plane when looking at them as a "C" and an "I" - this is due to the two beams possessing identical moments of inertia. The I-Beam will be more stable though, and will be superior in long-beam bending when flange crippling and column buckling overtake pure "strength" as the failure mode.

However, when bending them transversely to the above stated case, bending along a horizontal plane when looking at the beams as a "U" and an "H", the "U" will be much stronger as it will have a larger moment of inertia.

Also, a "C" or a "U" will be stronger in torsion than will an "I" or an "H"