The answer depends on the quality of the equipment you have.
If you have a very good sound card with S/PDIF out ( Lynx ? ), you're better off using that and a good cable.
If your sound card is ordinary, you have to use USB but you will need a good-quality cable ( cryo treatment is nice ) and a decent DAC.
if you need a USB-to-S/PDIF conversion box on top of that you'll need the S/PDIF cable too. ( 1.5 meters long, essential. )