Fool's Gold is very much the Minion of treasure cards: very strong if you can spam it and get multiples in a hand, but not worth much if your deck is dilute, whether due to lack of trashing, or junking attacks, or multiplayer games with more competition for the key resource. Get six of them, and you're usually sitting pretty; get four and they don't do all that much for you. (Minion is, of course, much less useless when you don't have so many, but it still suffers.) These are cards which often force a race for them for denial's sake, even when there are other good options which would in a vacuum be better than just getting 5 Minions or FGs.
So... what happens when they're both on the board? Which do you choose? You obviously don't want both, they work at cross-purposes. Here's a game where I chose Minion, and my opponent chose FG:
http://dominion.isotropic.org/gamelog/201209/17/game-20120917-201720-062108cb.htmlcards in supply: Crossroads, Develop,
Fool's Gold, Hoard, Inn,
Minion,
Oasis, Scheme, Walled Village, and
WorkshopI tend to think of Minion as stronger than FG, and it was here (I won with 6 Provinces on Turn 16), but in retrospect the real key was that Minion support was better. Workshop is in fact a reasonable enabler for Fool's Gold, especially when you're getting them uncontested, as a way to get more FGs in. My opponent, in fact, took the whole pile, whereas I only had 5 Minions. However, it's not a top-tier enabler... and Oasis
is really good for Minions- providing cash and cycling, and not really any downside (who cares about discarding Copper when you're going to use Minion for draw-and-discard anyway?). I was frequently getting more cash from Oasis than Minion, in fact, which tended to take on more of just a cycling role. And obviously Develop is a really slow and weak trasher, but turning Estates into top-decked Oases was pretty nice.
And, of course, the Minion attack hurts Fool's Gold pretty badly, since it cycles green in faster and makes it hard to line up three FGs in a four-card hand.
But replace Oasis with, say, Council Room, and Fool's Gold quite possibly comes out on top.