I'll second the USB output as being best, but it's by a very small margin - probably within the limits of being repeatable. I say it's best because asynchronous USB doesn't have to bother with a high precision clock in the player itself - the timing comes from the DAC. So if your DAC has a good clock, there's nothing to worry about.
I actually do use the coax output on my N100SC as the primary output, but only because the USB input on the connected processor isn't terribly reliable. That has nothing to do with the Aurender, of course. But the end product is still very good using coax instead of USB. Perhaps there's a more significant difference higher up the line, but I'm personally not likely to pay the price of entry to find out.