My take on it, for better or worse, goes something like this: flood plains are above average tiles, so you want to be working them all the time. Which means not pushing into an excess of food for your stable configuration.
So I'll usually prefer, for instance, 4 cottaged flood plains (12
/ 4
+ cottage yield) to three farmed flood plains (12
/ 3
), and idle flood plain, and a specialist/brown mine.
Perhaps I shouldn't: this analysis ignores growth rates, invested worker turns, and the possibilities of the whip.