I've read that optical S/PDIF can result in more jitter than coaxial S/PDIF, but never seen any proof of this. But this has led some people to prefer coaxial over optical.

The nice thing about optical is you're guaranteed to decouple the source and receiver. But a well designed coaxial input will do this anyway.

I think any optical<->coax converter would have to be an active device. And could increase jitter even though the digital signal itself would be the same.