I thought dampness is more of how a board feels more so than how a board is actually riding. Even though a board may be getting bump around by imperfect snow, you "feel" more stable on a damp board since you don't feel the bumps.
At least that's how I understood the "dampness" characteristics of a board.
If not, which board will feel more "damp" and stable from my original question. One with longer effective edge, or stiffer board.
Stiffness nor effective edge length play much of a role in terms of dampness. Dampness comes down to the construction of the board. In general, stiffer boards tend to be damper, but that isn't a rule carved in stone. For example, the Capita Black Snowboard of Death is a pretty stiff board but isn't damp at all. Never Summers tend to be super damp regardless of flex.
As to your comments on dampness, feel is reality. If you feel more stable, then you are more stable. A damp board will definitely feel more stable over bumpy terrain as the board is absorbing more of that shock and vibration leading to a more stable ride. Some people prefer that feel, others don't. What some interpret as stable, others interpret as dead and lifeless. Different strokes for different folks.