All other things being equal, an async USB link allows for better jitter management than an optical S/PDIF link.
Things however are rarely equal. It comes down to many factors, jitter mitigation in the DAC - perhaps it does a really good job with optical, galvanic linkage to the PC - maybe the PC has a noisy ground which gets exported down the USB link or perhaps the design of the DAC is poor in the first place .... and so on and so on.
All of these factors could favour one input over the other irrespective of the quality of the link or source of the digital data. My guess is with a decent DAC, you won't be able to tell the difference between your two proposed setups if you conducted a blind test.