[IROC]

I'm going to go with Occam's answer and say that they would be both as likely to knock it over. The main thing is the kinetic energy of the projectile (mass times velocity squared). The piece of wood isn't going to know the difference if the kinetic energy is the same. Sure, there may be some effects right at the point of impact, but it seems that conservation of energy is going to come into play and only the duration of the impact energy would be affected, not the total amount of energy imparted.

Is it balsa or oak? :>)

As long as the lead bullet does not exit the other side, they're essentially going to be the same. Picture a piece of plywood in lieu of the piece of firewood. The lead bullet would probably exit the other side and not impart all of it's energy into the wood, where the rubber bullet (being larger) will most likely not penetrate and impart more energy into the plywood.

I'm probably wrong, but that's my 15 seconds-worth of thought...

Mike