Big difference...not sure of all the technical issues involved but it's NOT like the old Optical vs. coax argument.... I am using an IMAC and was very disappointed in the Optical output quality. The USB into a good USB DAC is absolutely the way to go. Further, With the IMAC at least, Your use of the Optical adapter/Headphone port mutes the speaker output on the computer...you may want to hear other things/on other sessions without having to disconnect the Optical cable. There are several great USB Dacs available...including the Wavelength and the PS Audio Digilink 3... with apple lossless, the quality is, in my opinion, outstanding.
