The toughest material in Conan Exiles is either reinforced stone or black-ice reinforced wood. Conan Exiles' building materials are split into tiers: T1, T2, and T3. As these are both T3, there is no difference between them as far as their hit point pool goes. Check out their mats and see which you'd rather farm in your area to determine which will be the easiest to build. If you are in a harsh environment temperature-wise, then you'll need to consider which insulation you need. Stone is best in hot environments, and wood is best in cold environments. There are many other T3 building materials in DLC content.