I'm not a pro audio guy, but I think if you want to work with any external audio device and want that connection to be noise-free, your safest bet is still optical.
I have an external DAC/amp, and I had been using optical until I recently upgraded my workstation. Now I am using USB, because the new motherboard doesn't have optical; I assumed USB would be fine, so the lack of optical wasn't something I considered with the purchase.
Unfortunately there's an awful amount of hiss with USB on this DAC. I'm not knowledgeable enough to pinpoint what's causing it, but standard remedies I've found (eliminate ground loop, try ferrite cores, get a usb filter) have not made a difference. It could be the USB interface on the DAC side, I have no idea.
While I don't _need_ an external DAC, I like using one for a few reasons: mine has a low output impedance which is good for some headphones, a convenient switch to toggle between headphone and speaker output, and a big volume knob with really smooth action. I like it. The alternative is to use the motherboard's ports; this motherboard does jack sensing such that the rear line out gets disabled if something is plugged into the front, so switching between headphones and speakers involves plugging/unplugging the headphones all the time. I also don't like adjusting volume through a tray applet.