Listen to Wade.
Lightest AND smallest diameter.
Taking two rotating parts of the same weight lets say a solid axle verses a larger diameter gun drilled axle the larger diameter part of the same weight takes more energy to accelerate to the same speed. In fact a lighter part that is significantly larger in diameter can take more energy to accelerate.
Here is some casual reading on the subject;
http://en.wikipedia.org/wiki/Moment_of_inertia
Now when it comes to aluminum verses steel spools I think there is much more too it than rotational inertia. I think flex and friction may play a big role.
So just build your whole drive train out of titanium then we can start talking about $200,000 stockers LOL