Re: difference silverplated - goldlaquer mark VI
The color (and maybe the price/value).
Logic tells us that as gold is more dense - that it would produce a warmer sound than a silver plated horn (it really has to do with the additional mass on the outer layer - not so much the chemical composition when comparing metallics), but what really matters is thickness (weight) of the layer applied, the size of the horn (a more pronounced difference on bari than alto, for example).
Mouthpiece selection will far override the character of the sound compared to what body finish or resonator is on the horn.
Basically, get what you think looks nice if you are faced with the decision.