There's not a significant difference in conductivity between copper and silver. Silver does have an edge, but it's very slight (242 btu/h*ft^2*defF*ft for silver vs 232 for copper; for comparison, gold is 196 and aluminum is 130). Copper's so common because it is so well-suited - 96% as conductive as silver for a lot less money, so it's easy to upsize the conductor a fraction and achieve the same performance with copper as silver at a much lower cost.