Flexible is what you want, you just have to support the outter sheathing along the wayclose to the servo (no more than 1 or 1.5 inches away from the servo), at the firewall, and at least one or two places in between. I like flexible on throttles since you usually have to go up, down, left, right, whatever to get around the fuel tank, battery, and whatever else you have in the front and then out to the carb that is located God only knows where.....