When you compare stiffness there are many aspects that have to be considered. In every case sandwiched construction is lighter and stiffer than uncored structures. The final product is going to be as follows.
The bed will be Advantech Flooring which is dense, stiff, and dimensionally stable. More mass in the base is desirable as it absorbs vibrations.
The bridge will be cored aluminum skinned structures with a total of 1/2' of aluminum skins and 1.5 inches of end grain western red cedar. Some people have expressed concerns about thermal expansion differentials but this machine will be in a controlled environment.
The YZ plate is 1/2 aluminum, and the Z Plate is 1/2 aluminum.
The construction is coming along well. Using aluminum plating for the skins is good, as you all know it utilizes the natural flatness of aluminum extrusions.
We have a lathe, a mill and a shop full of industrial tools. I really have to give credit to people that build theses things in a home garages. You can see why so many of them are built using T Slot extrusions.
I will be posting some pics soon.