Most of the wood spacers are laminated, so they are not porous. But I have to ask. Are really serious? The difference if any at all between the two would be so minute you would be splitting hairs. Just pick one and go.
X2 to the above. I would stay away from the spacers/insulators that are stacked gaskets and aluminium plates. They can lead to cracked baseplates if you dont pay careful attention to your tightening sequence/torque.