RCA connectors don’t have a 75ohm characteristic impedance anyway (unlike BNC, which are available in several variants… ) whatever cable is used, so I reckon it’s a moot point. If you’re being picky, buy a DC1; if you’re feeding it from a tv, maybe optical is a better compromise.
Coaxial digital cables for SPDIF have a characteristic impedance of 75 ohm and are designed as transmission lines to match the connected devices.
Audio interconnects will have a wide range of impedances and are not designed as transmission lines.
However SPDIF is a very forgiving standard, so it is likely you can use an audio interconnect over a short distance as a digital interconnect, but you might get the occasional drop out.