The problem with the metal shed is that during spells of cold weather the thermal conductivity of the metal means it gets really cold. Then, usually in this country the cold weather is followed by mild weather with high humidity, the water then condenses out on the metal surface until the metal warms up, after a frost it can sometimes fall like rain on anything in the shed, you think that there is a hole in the roof, because in some places the water dribbles down an edge and build pools of water. Wood gets just as cold and no doubt condensation forms, however as long as its not painted with water repellent paint, the humidity is absorbed into the wood.