There are several big factors which make me go for farmland over duchy even in the copper case:
1. As noted, farmland -> province is another possible 3 point swing (without the farmland I'm buying a duchy). Odds that I will hit 6 with a farmland should be higher than hitting 7 with the new estate.
2. Most TfB options work better on the farmland/estate than the duchy/copper. Only expand can flip duchy to province; having 1 more value to feed into apprentice or salvager can also be better; additionally using the estate as feed (e.g. expand estate -> duchy, apprentice it for a lab) can put you over humps that would otherwise cost a duchy, cost a silver and lower a later VP buy, or simply not be possible.
3. Let's say I do end up sacrificing a province buy later on. I then grab a duchy and am down 2 points; I've lost very little. ~50% of the time I'm already going to lose the tie and will have to play for duchies/farmland anyways. However, with the extra point I can, perhaps 3 pile with an additional duchy or farmland, or play for farmland & duchy (say remodeling a lookout -> duchy) which I need to do just twice more to make up for 4-3 province split.
4. Aside from Tfb, there are considerations like menage, harvest, fairgrounds, baron, farming village, venture, etc. that like estate/farmland over duchy/copper.
My default would be something like consider gold first. If I'm not in the market for gold, then farmland & estate should be better than estate in a lot of cases.