Excellent advice above. I'd add that more recent USB models that take advantage of current USB>DAC technology are a significant step above previous implementations of USB. In the case of the later the USB would sound markedly inferior to the direct SPDIF from a transport. With current technology you'd be hard pressed to tell the difference. So if both are important to you, look at more recent USB DAC's. Empirical is a good suggestion. Other's that have very good implementation of USB include Wavelength (some of his DACs are USB-only though so be careful if you want to run a separate transport), Wyred4Sound, Bryston, Burson, PS Audio, and others. You'd really need to pick a budget and then start listening to how some of these DAC's sound as they don't all sound the same. That choice is entirely subjective.
Is there a qulaity DAC that has USB and coaxial.
I wuold like to use one DAC with CD, and computer USB. Is it one or the the other? Am I missing something?