@florrat:
Of your choice means that when you gained the card, you gained it using an effect that had at least 2 possible cards that you could gain as a result of the effect. So if all the prizes are gone, gaining a Duchy is not of your choice. If prizes remain, the card you pick is of your choice, so you probably want to pick Duchy lest you gain a curse.
Trader doesn't create an infinite loop, because gaining a Silver due to Trader isn't "gain a card of your choice" event, it's a "gain a Silver" event, which does not involve any choosing. You choose whether to reveal a Trader, but you don't choose which card you will gain when the gain event is replaced. So the Expedition doesn't trigger and the player to the left can't pick a new card at that point. You'll get a Silver.
Cursing attacks, Cultists, and even Explorer all give you a specific card, so you get that specific card, because you don't choose what you gain.
The card was supposed to actually say "gain a card of your choice or a card you bought", but I messed it up and don't want to violate the finality rules. Buying a card causes you to gain that specific card, you don't get to choose which card you gain as a result of buying, it's always the one you bought. So as I currently have it written, the effect will only trigger on Workshop type events. Possibly this error is some of the source of confusion, since it is easy to tell that the designer's intent is for the card to work on buys, but a literal reading of the phrase "of your choice" makes it not work on buys.