I would find out if the Oppo and the Sony are fully balanced. If they aren't balanced, they should sound just as good on the rca inputs.
From the Oppo website page on the BDP-105:
The balanced output features a true differential signal path all the way from the DAC to the 3-pin XLR connector. By transmitting a pair of differential signals, the balanced output provides better common-mode noise rejection and improves signal quality.
Also, I would very respectfully disagree with ZD's second sentence that I quoted, even though what he said is a commonly stated belief. Balanced vs. unbalanced interconnections potentially call into play a lot of differences that are unrelated to whether or not the components have balanced internal signal paths. Including ground loop susceptibility (which can affect high frequency noise and background blackness as well as susceptibility to hum); differences in the quality of the interface circuits of both of the components that are being connected; cable differences; sensitivity of the components to cable differences; substantial impedance differences in some cases; and possibly noise pickup in some cases. Perhaps even differences in the amount of attenuation that may have to be provided by the volume control, in the common situation of a 6 db difference in system gain between the two interfaces, would have audible consequences in some systems.
Which is not to say that a balanced interface will necessarily be preferable to an unbalanced interface when connecting components that are not internally balanced (although in theory a ***well implemented*** balanced interface should be better). My point is simply that differences can be expected.