West Central Alberta Coldest, wettest edge | The distance between the top and bottom plates will gain you the most( by memory, I think the distance relationship is squared,whereas increases in area have no multiplier, 1 to 1), but since you are keeping that constant, and only increasing the width, you will still get more strength from the 12" wide. It will take 1.5 times more effort to stretch the bottom flange, and 1.5 times more effort to compress the top flange, due to the increased area over the 8". Which is exactly what Exit is referring to regarding mass moment of inertia.
Image is of an I beam, but the principle is the same( in the up and down direction), just have two webs instead of one. The formula at the bottom, I subscript X, is the one you would use, since the webs (verticals)are constant, regardless of width, you don't even need to calculate that, if all you want to do is compare. A bigger moment, means stronger in that direction.
Edited by Alberta Farmer 2/23/2013 01:00
 (area_moment_of_inertia_I_beam_example.jpg)
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Square vs. rectangle? Tube bridge physics.
