The simple answer - just the exterior ply of material. Unless noted on a specific product, all of our carbon fiber tubing is made with UD prepreg on the inside, and then will have either the UD exterior ply or a fabric ply. The twill carbon fiber fabric exterior is a popular choice as it provides the carbon fiber "look" that so many customers associate with carbon fiber.
Yes and no. It depends on the wall thickness and which property is of interest. If the tube wall thickness is the same, technical speaking, the UD exterior tubes will be slightly stronger and stiffer in bending (less than 10%). However, the hoop strength of the fabric exterior tubes will be slightly stronger. Again, this strength or stiffness increase is only a small percentage and will not be noticeable without precise testing equipment. It is advised customers choose between these options based on cosmetics, not strength. Another consideration is if holes are to be drilled into the tube, the twill exterior will resist splintering better than the UD exterior.
When designed for bending, carbon fiber tubing is typically around 1.5X stiffer than aluminum, assuming the same dimensions (diameter and wall thickness). This will though depend on the carbon fiber grade and laminate design. For example, high modulus carbon fiber can be designed to be over 3X stiffer than aluminum in bending.