I would say the Asus card hands down. Advertised numbers are not always realized, so it really not the best idea to go by them. But, you can look at signal/noise ratio, number of outputs, bit rates, etc. Steelseries is unable to put as large or as high quality parts into that small of a case. I would take a quality PCI/PCIe card over USB audio almost every time. Asus makes great cards too as I have used a few of them and prefer their sound over onboard solutions.
i can say and garantee the asus dx has better quality, the specs also can be misleading, i cant find a headphone that has better quality built in, unless ur talking of high end headsets with amp thats a different story.