Obviously the monetary value of an IGG that you use the Copper-gaining power of is (normally) equivalent to that of Silver, and so it's tempting to say that the Silver version would be superior, since you don't have to pollute your deck with Copper to take advantage of it.
However, what the real IGG does for you that Silver doesn't is provide resilience to your deck through the greening process. IGG/Duchy works because you can buy tons of green cards without clogging up so much, because the influx of Copper dilutes the green. With Silver, your deck will falter quicker.
So I'm going to guess that IGG is better only when your target $ amount for greening is $5 or less. Then again, I suppose if you are pursuing a rush strategy with either, it would be.