That's funny, I asked that on page 18:
Silver as a general rule gives you a more accurate sound reproduction, this can tend to sound brighter and thinner especially if you have a budget system. Copper can give a more coloured sound and can give the effect of warmth and more bass. Silver/copper is a bit of both. I've always favoured SPC or silver/copper.
All depends on what you think your system would benefit from.