Another vote for the red handled splitting maul that Richard_ posted.
I also have monster maul, but it gets awfully heavy after a couple of hours of swinging and makes my shoulder ache!. No only can I work longer with the lighter tool, but the physics are in favor of the lighter tool as well.
F=MxA. Force = Mass x Acceleration.
A=M/S^2. Acceleration = Meters per second squared
So, doubling the mass only doubles the force. Doubling the speed of the head quadruples the force.
This assumes that the acceleration as the head impacts the wood is consistent (which it's not), but it is still close.
The other issue is that the optimum head weight is different for everyone. The ideal head weight would be the heaviest that you can still swing as fast as you could swing a lighter weight. If you can swing a monster maul as fast as I can swing my six pounder, I have no problem admitting that you are the much bigger man!
Adam