[bellsprout]

But in the absence of info about the design of a particular DAC, can you make the assumption that optical in is more likely to be implemented correctly because it's a more tried and true technology? I haven't seen many reviews that measure jitter performance of various inputs (actually I haven't seen many reviews that measure period, but that's a different point)

 
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It all really depends on the quality of the usb input on the dac. Asynch usb has made it a good choice more often, but it all comes down to the design of the input. Does it isolate the 5v line and power the usb chip from the dac, does it reclock it, is it asynch, does it support higher resolution. These are all things that can help make the usb better option if they are implemented correctly.
 
The best thing to do is try it both ways with your dac and see which sounds better. If you cant tell the difference, than count yourself lucky and do whatever is easiest.