Size Matters: Bigger IS Better
Compare fairings' total area, and in particular compare length (longer is better), distance from ground (deeper is better), and the leading edge (is it full and square, or angled; full and square is better).
Shape Matters: Different shapes yield different savings
Is the fairing parallel to the side of the trailer? Straight, but installed at an angle with the front inset? Parallel to the side of the trailer, but toed-in at the front? Parallel, with an inset leading edge, is best