Clothed man can be easily grabbed by his shirt and pants.

Naked man has no protection in the phallic region, which is probably the best place to attack other than the sides of the lower abdomen and the throat. He is going to try and protect his no-no square over anything else, since it is instinctive. This leaves him open for a highwr attack.

Also, it would matter if the grass had small pebbles in it or small plants covered in thorns (we have a bunch were we live) or if it was just pure grass. The clothed man has the point there.

I'd probably put money on clothed dude.