Like with most materials, it depends. If you want rigidity, then you will want to use FTX Green. It is great long-span bridges and delicate parts, whereas FTX Cast is ideal if you are looking to burnout multiple single units. The FTX Cast material melts much like a wax and reduces retention on the investment.
However, you are unable to fluidly move between using the two materials as it does require changes to the software settings. It is recommended that you choose the material that best fits your strategy and desired results. Below is a side-by-side comparison of the material properties.
Material Properties Overview: