A blue foam will be MUCH lighter. the frame we designed for our client was origionally cut from structural urethane foam for its core. the core alone was 2.5 lbs which didnt leave much room for carbon. the blue foam core was .75 lbs. the "structural" cores are designed for sandwich laminates. you are not doing that. you are covering a shape. here's my analogy: if your final goal is a 10 lb structure. would you rather have a heavy core and not much skin, or a light core and 4 times the skin? theres more strength and stiffness in the additional carbon, if and only if you have a one piece design, or a solid core. the blue foam allows you to make a more similar construction. more laminate weight less core weight. they are talking about shear strenght etc....flat panel construction uses cores to give a "3rd" demension to the construction for rigidity. that rigidity depends upon the shear strenth to hold the sandwich together. this is where the designed cores shine. you are not constructing like that. you are making a SOLID shape and covering it. completely different. you will end up with a more "one piece" construction as our bike frame. boat hulls are made in multiple pieces and assembled to some degree, are very hollow, and are resonably flat. so the walls need the structural core to keep rigid. your "solid core" construction does not follow that ideal, so your choice of materials needs to follow your need, not generic construction theory. just my very humble opinion