RCA or Optical?

Which sounds "richer"?
Does not matter if working properly.
Does not matter if working properly.

Does not matter? The audiophile police are watching you.
I've used both over the years and NEVER been able to tell the difference.
I've used both on half a dozen Dacs and have found even a cheap RCA sounds better than a big-buck optical.
I recently reconfigured my system and came up short with my vintage Audioquest RCA (true 75 ohm) digital cable.
I substituted a generic Toslink and thought there was substantial improvement. I purchased a glass fiber Wireworld Toslink. I like it but can't get it to pass 24/192 data to a Rega DAC (it's 3 meters long BTW).
I've had well over 50 DACs in my system over the past 4-5 years.

I have never heard an optical (toslink) connection that even comes close to what a coax can do. Nor a coax over what an AES/EBU can do. Nor what a I2S or Firewire can do over the AES/EBU. Although very few devices have I2S or Firewire connections, but the ones that do are superb.

My order of preference:

1. I2S or Firewire
3. Coax (BNC)
4. Coax (RCA)
5. AT&T Glass Fiber
6. Toslink

Just my opinion...
Optical does not pick electrical noise or create one (no transmission line effects) but transitions are slow making it more susceptible to system noise, affecting threshold point and creating jitter.

Coax has faster transitions making it more immune to system noise but is sensitive to ambient electrical noise (needs good shielding) and transmission line effects - characteristic impedance of the cable has to be matched to one of the system to avoid reflections inside of the cable from impedance boundaries. Such reflections affect shape of transition hence level recognition point (threshold) resulting in jitter.

Because of all that digital cable is a system thing - what works for one system might not work for another or even exactly same system in different location.
+1 Kijanki. I have a very long run (8'+) from my digital cable box to my processor. The box had a bad SPDIF output, so went with an armored TOSLINK. Works perfectly. On the short run from my BD to processor, I found the TOSLINK far more listenablein my Mc gear. Having fun and happy listening are the goals, after all.
I use 2 AQ coax...VDM3 from the CD player to the DAC, and a VDM5 from my
streamer...they both work very well, and although they're different lengths and
designs (the "3" is silver coated copper, the "5" is solid
silver) they are absolutely indistinguishable when switched, which means they
suck evenly, or they succeed evenly, or I'm an idiot (most likely).
Optical are a pain to plug in behind a rack....