I had both cables on my Sony 999ES. Like most other comparisons, I believe results are very system dependent.
What sounds great on my setup may or may not produce similar results on other systems.
That said, I'm using Innersound Eros electrostatic speakers which are so clear and detailed that even small changes
are noticeable. The silver umbilical is anything but harsh and bright. It is sharper on the upper frequencies and a little
less 'full' on the midrange. The copper produces a better midrange on my speakers, with slightly less detail and separation
on the upper frequencies. All in all, differences are not earth shattering and both cables perform well.
I noticed a much larger and noticeable improvement by switching to Bendix 6900 tubes.
I'm now running Dan's new modded 9100ES with copper and tube rectification. That's a whole different story