It's a tough decision indeed. Moving 1W adds the unhealth, as mentioned already. And the extra 1 flood plain doesn't become a factor until the city becomes large enough to work it...which is well after the point when the unhealth kicks in.
The question really boils down to this: what are you willing to sacrifice in order to have a Gold in your capital's BFC? Without any visible food source with >4 food in sight, it will be a hard effort to provide enough food to work the Gold. Without the burden of working the Gold, you could otherwise work 2 flood plains cottages instead of farms...as the cottages mature, they will eventually make up for the lost commerce from having no gold (assuming that you want to keep the same level of food surplus).
Another way to see it is ask, how would you otherwise work the Gold? 1W of the Gold is one option, providing a plains hill, although missing the fresh water bonus. However, such a city would have only 3 flood plains, and therefore less unhealth.