Depends what finish they use. Bright silver on cassettes is usually nickel plating, which is used for corrosion protection and might offer lower friction than the base metal. The ED black is also a corrosion inhibitor, but it's softer than nickel plate. In practice, on a heavily used bike neither will make much more than a cosmetic difference.
a nickel plating might provide a higher hardness and thus provide a slightly better resistance to wear but I agree with you on the fact that on a heavily used bike this would not make much of a difference
Depends what finish they use. Bright silver on cassettes is usually nickel plating, which is used for corrosion protection and might offer lower friction than the base metal. The ED black is also a corrosion inhibitor, but it's softer than nickel plate. In practice, on a heavily used bike neither will make much more than a cosmetic difference.