with ufh it must be flexi as you will get differential thermal expansion and contraction and uncoupling is a specific recomendation of the british stone federation. i agree with widler... go with the one who says flexi and uncoupling as he at least sounds like he knows what he is talking about. slow or standard set means you would not be able to traffic the tiles or grout them for at least 24 hours. i dont do much tiling myself and then only for myself but even so i prefer rapid set