It's not as simple as one interface being better than another. There is a lot going in with computer audio and there are good and bad solutions for both. If you already have a good DAC with Toslink input, my recommendation is to start there. Get a good Toslink cable, such as the Wireworld (you'll need an adapter for the mini Toslink jack on the Mac) or the van den Hul, which you can get with a mini Toslink on one end. If your DAC has a USB input, odds are it sucks. To get good USB, you need one of the specialty USB DACs or one of the speciality converters (converts USB to other interfaces), such as those from Bel Canto and Empirical Audio.
In my experience, there are no clearcut answers in computer audio (heck, in audio in general). I recommend experimenting, if that interests you.
