What you plan to do while wearing your corset will impact this decision. A stiff busk will offer more support, but will also hinder some movement while laced. A stiff busk is generally better if you prefer maximum support, and do not plan to do much bending or sitting. However, a stiff busk can work on a shorter corset to still allow more freedom in range of movement and ability to sit comfortably for long periods of time.
A flexible busk will offer slightly less support, but will allow for more movement. Unlike the stiff busk, a flexible busk will contour your side profile view. (bend out ward at the rib and hip, and inward at the waist giving a concave profile). I feel a flexible busk is easier to stealth, as it gives a more human shape, and is more comfortable for longer and active days.