Best material for speaker cables?

It depends on your system (and room), how the cables are designed and constructed, and last but not least - your personal taste. I can see no answer to your question other than auditioning both types, yourself, in your system. I have not tried silver speaker cables at home in my system, I do however use both silver and copper IC's and like both materials, it just depends on the cable.

Yanomami: My speaker cable runs are 17' which is another reason that I will not be trying silver cable anytime soon. Another option other than silver and "typical" copper are the micro wire designs such as those offered by Mapleshade. I use there Double Helix digital cable (micro wire) and it is very musical in my system with abundent detail. Users of their speaker cable feel the same way. They are however limited (in normal use) to 12' runs. I may try them with a SET amp that will eventually end up in a smaller system with shorter speaker cable runs. The micro wires are available on a 30 day trial basis if ordered in standard lengths of 8' or 12'.