If hitting $6 as early as possible is the only thing you care about, I think Duchess-Silver is slightly better than Silver-Silver, since it could potentially let you skip an Estate early on. But, this is obviously bad strategy longer term.
Of the options you suggest, I intuitively would think Silver-Silver would be faster than Silver-Plaza, but I might be wrong. The extra Silver gives more coins per card, but the Plaza allows you to save a coin if you don't hit $6. But, it requires you to wait at least 1 turn to do so.
I agree that both options are probably faster than Silver-Marauder, which needs a reshuffle before it can hit $6. But, this might be the best option on this board anyway for the attack.