Its quite hard to model these different designs without being able to calculate the moment of area. But to answer your last question, the L-shape created by two sections of extrusion, assuming they are rigidly joined will be slightly stiffer than the single piece in the X direction but two 45 x 90 in the X direction will be stiffer again.. without doing the math, I'm guessing the two 45 W extrusions bolted to a piece of plate in Jazz's design will be slightly more rigid in the vertical than the 45 x 45 bolted to a 45 x 90 horizontally. So on balance the yellow one I'd say is stiffer then the green one, certainly for static forces vertically or horizontally on the gantry, but it weighs more. Torsional stiffness is harder to assess. Box section is far better than extrusion in resisting torsional forces, but my gut feel is the green design is likely to be better than the yellow in this respect...