Horse Traders is pretty darn good for Duchy/Duke, as it lets you get to five quite often.
It sounds appealing, but is not really that good, because to make a duke/duchy strategy work, you need to do more than "get to five quite often". You need to get *multiple* duchies per turn, or your opponent will just respond by buying half the duchies and be just fine.
The most likely card to make duke/duchy work, imo, is horn of plenty. Get a bunch of horns (using them to gain each other), and then turn them all into duchies, not giving your opponent a chance to block your duchies. Since you haven't committed to the duchies early, he can't really do it pre-emptively, or you can just switch into something else.
I think the focus on duke vs duke strategies is mostly red herring. I am not sure I've ever seen a duke vs duke game, it's almost always a province vs duke game when at least one player goes for duke/duchy. So presumably as a duke player, you should be able to handily beat your opponent in the duchy race (opp went from provs, after all). As the duke player, you want to be able to consistently churn out 5s for a long while. While most province decks are optimized to churn out 8s for a short period (just enough to hit 4 provs and change).
