Compared to other Gold gainers like Explorer, Dismantle and Bandit, Bag of Gold is a poor choice. Sure, Bag of Gold is nonterminal, but Governor provides an additional junking effect which can ruin some combos like Vassal, Baron or Scrying Pool. The topdecking á la Artisan is more a disadvantage than an advantage, because it can actually slow your next turn down as you may miss an engine part. Like Bureaucrat, its a good card for Gardens slogs with large decks, though if Trusty Steed is already taken. Explorer is so much better, because it gives you the Gold into your hand, making +$3 instead of +1 Action. Coincidentally it has a similar Province requirement like Tournament, but if you already have a few Provinces, the +$3 is much more valuable than the Gold. To conclude, I would always skip BoG for a Duchy.