There is no absolute on determining which type of cables is superior. It depends on the quality of the products and the system matching. The Copper cable can sound a bit smoother and warmer in general, while the Silver cable usually sounds faster and articulate. One thing about Silver cable is when it gets oxidized, the conductivity is still not too bad.
About the mixing, as long as you like the sound, it really doesn't matter that much. However, the matching can be somehow more difficult than using the same type of cables.
For the bi-wire or bi-amping, if you really need to mix, we prefer silver or silver plated wires for the woofer and copper for the tweeter. The tweeter can sound a bit smoother while the woofer can sound a bit faster.
Good luck.
About the mixing, as long as you like the sound, it really doesn't matter that much. However, the matching can be somehow more difficult than using the same type of cables.
For the bi-wire or bi-amping, if you really need to mix, we prefer silver or silver plated wires for the woofer and copper for the tweeter. The tweeter can sound a bit smoother while the woofer can sound a bit faster.
Good luck.