Most Difference

I hate to say this… so predictable. But, it depends. 
 

Typically my SP/DIF have exhibited pretty high sensitivity. Except for that a couple times where Ethernet cable that made a bigger difference than typical. 

Sorry. 

@ghdprentice +1

too many unknowns in this equation.
OP can you provide a bit more context? System/components/available connection options, current ethernet set up?

The spdif is more impactful

Ethernet is the way to the present and future. 

Network Acoustics Muon Pro Ethernet cabling system. Game changer

I am getting the Authentic Audio Image Assoluto ethernet cable lets see how it turns out and what results I get

Has anyone used their cables(AAI) Jay's Audio Lab is the distributor in USA I guess and that guy is into serious gear

Let us know how it turns out. 

@jasbirnandra - as someone else posted - too many variables - but here’s a few points that might sway you one way or the other

ETHERNET - designed for data transfer from the ground UP

• long runs can be accomodated with ease
• CAT8 is a very good low noise design that can achieve exceptional throughput,
  • I did not find much difference (if any) between CAT7 and CAT 8 on the devices on which they are used
  • my house wiring is CAT6A and appears to work very well
  • my CAT5 cables by comparison were poor and have long since been replaced
• the interface has been established for decades, so the interface in most components tend to provide very good results

SPDIF (optical) - designed from the ground up for data transfers

• performance depends on the material used for the optical medium
  • plastic is more affordable
  • but glass is better and more expensive
• the SPDIF interface in a component can be a weak link
  • so even if your DAC has a great SPDIF interface, the interface in an attached source might not be quite as good (or vice versa)

My preference is for ethernet over any other method of digital transfer and as such, I have eliminated other types of digital transfer across my whole household, with the exception of

• HDMI
  • my latest HDMI cables are "similar in nature" (i.e. cable geometry) to my CAT8 Ethernet cables, so I consider them to be as good as Ethernet from a performance perspective, which is reflected in the sound quality achieved
• WIRELESS
  • fortunately my house is wired for ethernet, so I only use this aproach when needed e.g. outdoor music
  • but with the right router, exceptonal quality can also be achieved

Hope that helps - Steve