We put more emphasis on the area of a reflector, excluding any supporting rim for more reliable like for like comparisons, that is, width x height / 2 or the diagonal measure & ignoring varying advertised dish sizes. That one proves more reliable for 'comparison' tests.
Channel Master obviously noted the area measure too as being significant as they used it on model numbers, several other major manufacturers variously use the area measure, not consistently unfortunately, but we can impose it across the board to compare like for like & avoid the various reasons why sellers & manufacturers vary it.
The offset measure - only measuring the width & assuming that as the height or diameter of a circle in the offset angle for signal collection / reflection & what the lnb sees is not quite as reliable if you compare reflectors of the same width but with different heights/ offset angles.
Some will steadfastly insist that the offset measure is the only measure for offset dishes, I think a little too preoccupied with the basics of the offset principle without looking at how signal is actually collected/ reflected & received into the lnb & feedhorn, but it is still far better than relying on seller's various size claims.
After that it's how well designed & manufactured they are obviously.