Frankly, my dear, I don't give a damn.
It just doesn't matter. RCA connectors (or 3.5mm coaxials) are what's usually used for S-PDIF, so that's what you get on most, even seriously (stupidly!) expensive cables.
BNCs are better, insofar as they can be proper 75 ohms, but then they get used with a BNC-RCA adapter
Unless your digital cables are
very long, and by that I mean over 10 metres (that's the longest I've ever used at home) anything will do fine. There may be some DACs that have such poor receivers that they might just be upset by non 75 ohm cables, but none I've ever used have ever been bothered by the cable. Try it, if it works, it'll work fine. The worse that can happen is that it won't work at all. That's one of the nice things about digital. It either works, in which case it works well, or it doesn't work, in which case you'll get drop-outs and mutes and weird noises. There's NO WAY it can
ever affect sound quality except to be muted.
S