Depends on the DAC or amp I'm using, but it doesn't really matter either way.
One benefit of the optical out, however, is that it keeps your USB ports free for other things.
I highly recommend the free utlity
SoundSource by Rogue Amoeba for use with USB audio devices. It allows you to use a simple menubar utility to direct where your audio goes without having to open up AudioMIDI Setup.
--Chris