[Dr G]

It seems unlikely to me that the metal would make a difference, but does anyone have thoughts about or experience with the performance differences between brass, copper, silver-plated or gold-plated resonators?

Welcome, Ken.

Material doesn't matter - size does. I haven't seen copper resos, but they will corrode even faster than bare brass, so I wouldn't go there. Plated resos will corrode less quickly. Color is up to your aesthetic preference.

I think nickel plate is most common, and that's what I recommend if you prefer to go with metal resos.