will I gain anything by connecting my spdif on pc to audio interface spdif ports?
Yes, you’ll be able to hear what you’re playing before you play it 
Thought it may be faster than usb? or more reliable
Comparing Apples and oranges…
what is SPIDIF:
What is USB:
The only advantages you will have, if you decide to use optical cabling, is to have thinner cables and the elimination of a potential hum source, because there is no more electrical contact between the devices which could cause grounding issues.
Thanks for info guys
Cheers
If it is the PC hosting your USB audio interface, no interest.