You want to Counterfeit one of them, Platinum is already in the Trash, Gold is not in the Trash, and you don't want to beef up your opponent's Foragers.
Also with Counterfeit, you want to put a Gold (or Silver) in the Trash rather than a Platinum, so that your opponent's Rogue won't be able to attack you.
Alternately, you want to beef up the Foragers that will be in your upcoming Outpost hand.
More importantly, you played Navigator this turn, so you know that you will have 5 Market Squares in your next hand. You also know (for example, from a 3rd player's Cutpurse) that your opponent has a Noble Brigand. You have no other cards in your deck or discard pile. You want to trigger the Market Squares, so you want Gold or Silver to be in your discard pile (and thus, next turn's deck), rather than Platinum.