I suspect that the correct model designation of your W4S integrated amp would be STI-500, not SSi-500. If so, it is described as a fully balanced design, and the following statements appear in its manual:
The line-stage within the integrated doubles as an unbalanced to balanced converter which provides a balanced signal from unbalanced inputs.
The Neutrik XLR (balanced) connectors are the finest balanced connector on the market, and are the best possible option for connecting your amplifier to your sources.
The description of your Oppo BDP-105 states the following:
The stereo output offers both XLR balanced and RCA single-ended connectors. The balanced output features a true differential signal path all the way from the DAC to the 3-pin XLR connector.
Based on those statements the answer to your question should be clear. At least insofar as which kind of interconnection is likely to provide performance that is most accurate, i.e., that is most true to the source material. Greater accuracy may not always be subjectively preferable, of course.
In general, in a home audio system my belief is that when choosing between balanced (XLR) and unbalanced (RCA) interconnections the designs of the interface circuits in the specific components that are being connected, and also the internal grounding configurations of the two components, are likely to be more significant factors than differences in the intrinsic characteristics of the cables that may be used.
Good luck. Regards,