It's entirely situational. Sometimes you have more food than you need and are clearly better off spending the extra food. Other times you don't have enough food and would end up spending more money for food at the end of the round anyways. Often it is in between these two extremes, and then it can be a tricky decision.
When paying to another player, that player's situation may be relevant, but it's generally secondary to your position vis-a-vis food.
What you need to look at is how much food you have compared to how much you need at the end of this (and future) rounds. If it will end up costing you more than one VP/money to get back the two food later (and you'll need), you should probably spend the money. Otherwise not, but even this doesn't cover everything.
Concrete example: You have 1 franc and 1 bread (and no other food), and want to use a building that costs one food. Before your next action, the round will end and you'll need 2 food for support. In this case, you can spend the money, and then the food at the end of the round, leaving you with no food or money for your next action. Or you could spend the food, leaving you with not enough money for food at the end of the round, which will force you to take a loan. So on your next action, you'll have 3 money, but also have an additional loan, a net cost of 2-4 VPs, BUT it may be worth it if you need cash to use a building on your next action and would otherwise have to sell a building to get that cash. Of course, if another player has an action before the end of the round, they might screw up your careful plans by using one of your buildings and giving you a fish, so you no longer need a loan...