You have to think about where the load is going. Most of the pressures involved in surfing are bending the board in half. That’s why you see most boards are buckled across the middle. The exception is with larger waves that will try to twist the board. This is where the bias layer will benefit (actually, all boards with a stringer will twist with the standard load, and benefit from a bias layer, but I digress). You don’t need a heavy cloth for the bias layer, as the twisting load is much less than the bending load — 4 oz. is plenty. The fibers running at 0 and 90 are the ones that prevent the board from folding in half. When you make the test panels, you will see how weak the bias panel is with the bending load and how much stronger it is in the twisting load. It’s been a while, but in my composite classes we determined that a bias panel had, I think, less than 10% of the strength of the 0-90 panel. If you want flex only, you can go with the bias, but if you want strength and spring, you need 0-90.
I think it all comes down to why you want to use carbon fiber, and I think the answer is, to make the board as light as possible. So, for me, it makes sense to use the carbon in the direction that it will give the most benefit, and that is in the 0-90 direction (actually just in the 0 directon, but I digess). Now, if your board comes out too stiff, then you should go down a weight in the carbon cloth that you are using. The added benefit, is a lighter board. But the drawback, is that carbon is not very durable. That’s why the layer of glass over it — fiberglass is a lot more durable. But if you add a layer of glass at 0 and 90, you are adding stiffness. You could compensate with a lighter carbon fabric, or you could put the glass on the bias. This will make your board more durable, light weight, and flex good. The added benefit is the board will be much stronger than regularly glassed boards. When you lay up a 0-90 and a 45, you are creating thousands of little triangles, and triangles can handle loads from almost every direction.