No matter what you read here, listen for yourself and then decide. This stuff is dependent on your equipment and on your environment, including the length of run from your router to your streamer, and the amount of EMI/RFI present in your home. The equipment I use (both the server/streamer and DAC) have internal galvanic isolation and I have yet to hear meaningful differences when trying a variety of small boxes that many others say have a profound effect.
That said, I am currently running 45 feet of fiber from my router to just before my server/streamer, where it is converted to a very short Ethernet cable, which enters the streamer. Both the converters, at the router and at the server/streamer, are powered with linear power supplies and not the standard SMPS used in wall warts.
I have run CAT 5, CAT 7, and CAT 8 cables and again have not noticed significant differences. If anything, the fiber may result in a bit smoother sound compared to any of the CAT cables, or it may be my imagination. You may want to look at the differences between CAT cables which vary in the level of shielding. CAT 8 provides the highest level of shielding and could offer an improvement in noisy environments. Both the fiber cable and the converters are inexpensive so try for yourself and let your ears decide what sounds best for your gear and environment.
That said, I am currently running 45 feet of fiber from my router to just before my server/streamer, where it is converted to a very short Ethernet cable, which enters the streamer. Both the converters, at the router and at the server/streamer, are powered with linear power supplies and not the standard SMPS used in wall warts.
I have run CAT 5, CAT 7, and CAT 8 cables and again have not noticed significant differences. If anything, the fiber may result in a bit smoother sound compared to any of the CAT cables, or it may be my imagination. You may want to look at the differences between CAT cables which vary in the level of shielding. CAT 8 provides the highest level of shielding and could offer an improvement in noisy environments. Both the fiber cable and the converters are inexpensive so try for yourself and let your ears decide what sounds best for your gear and environment.