Our group has several experiences of wheels up on grass. Since we fly amphibs and the prop is behind the wing and above the hull we present a smooth surface. Ususally there is no damage, Just jack her up, drop the wheels and motor away.
The braking effect of grass is pretty good, far more so than regular brakes. When I was watching a wheels up the plane stopped in about 30 to 40 yards.
A few weeks ago one of our group put his plane down wheels up on tarmac. He realised as he 'sank' just what he had done and elected to power up. Took him about 2oo feet to get enough flying speed to drag her off and left a whacking great scar down the bottom of his fibreglass hull . Somewhat surprising that he got enough speed and elevator authority to get the nose up enough to lift off.
(By the way, Wheels down on water wrecks it every single time.)
Of course if you are wheels up in a regular plane walking away is the point, personally I don't have a clue but emotionally I'd choose the grass.
Are there any statistics on this?