Mixing Fool's Gold and IGG sounds like a bad idea, so it's pretty much one or the other.
Points for FG: Herbalist can give consistency and early +Buy, Council Room can massively increase odds of FG collision.
Points for IGG: Has more flexibility in buys, because it doesn't require buying as many FG as possible as quickly as possible. So, it can fit in Lighthouse, Thief, and Fortune Teller more easily.
Both strategies benefit from Thief: If Thief hits Fool's Gold, that's good no matter whether the player actually wants FG or not. If Thief hits IGG, the player can gain it to hand out a Curse without having to spend a $5 buy on it.
I think I'd actually try the Herbalist/Council Room/Fool's Gold route, ignoring IGG. The IGG -> Duchy pile out strategy doesn't sound quite as fast, and if you pile out Fool's Gold then you can exert a lot of pressure with quick Provinces (because the IGG player has to start picking up Duchies a lot sooner.)