What do you mean by “speed dynamics”?  Do you mean Explicit Dynamics?  I understand that requires linear elements, but the implicit solver used for dynamics problems can work with quadratic tetrahedral elements. The benefit of tet elements is you can get a result quickly with a coarse mesh and very easily refine the mesh where needed. Contrast that with the time you will spend simplifying the geometry and slicing it up to get a hexa mesh, and how difficult it is to refine the mesh later. This is why I recommend a quadratic tet mesh.