I would not say that more money is not worth it. It all depends on what types of headphones or speakers that will be driven from the soundcard. If u need amping (headphones with 50-600OHM) they will get a great benefit and a much richer sound.
Still there is no point in getting anything other than the DX/DXS if you will use fairly easy to drive headphones(they use the same good DAC and have the same surround abilitys that you need for headphones)