Optical has much lower bandwidth than audio over HDMI as it can only handle 2.0/2.1 channels uncompressed, whereas HDMI can handle at least 5.1/7.1 uncompressed. Multi-channel audio over optical always has to be compressed.
In your case the optical from your TV to your AV receiver is already the narrowest link in the chain.
If your TV has ARC and your AV receiver can take HDMI in, then that would be the “better” option in terms of audio quality.
According to google and some other forums. HDMI is superior because it can handle more formats and more bandwidth. Not to mention the fact it does Audio/Video so no need to have seperate cables.
I ended up using the HDMI ARC port in my TV. So basically, Sky box to TV via HDMI, then TV to AMP via HDMI.
Watched a film earlier on the SkyStore and sounded like i was in the cinema
You also need to know the technology used, for example the PS4 Pro uses what is called HDCP 2.2 so if your 4K tv doesn’t support HDCP 2.2 You will only get HD quality, people had to keep swapping over hdmi cables when using version 1 of the PSVR.
I bought a virtual surround sound system back in 2008 which is now useless unless I want to just watch HD on my 4K tv