The game will only allow you to build certain amounts of each type of building as you level up your stronghold and open up new land. By the time you get to the end, you’ll have 9 farms and 4 mines. Your only real choices are which ones you choose to level first and which one you choose to turn into troop barracks.
That said, if you think food is more important to your progress than iron, feel free to focus on leveling your farms first.
I’ve noticed, as a general rule, that food has SLIGHTLY more uses than iron. But at your level (I mean general player level), I would focus on getting a training camp 20 up and running…for this you will need iron to level your ore storage, then your stronghold, and finally training camp…of course, it is best to stagger your builds e.g. ore storage 14, ore storage 14, ore storage 14, stronghold 16, training camp 14 (or whatever), etc. And as Dante noted, definitely build a barracks in place of forge (if you haven’t already)…in my own game, I always seem to get low on food b4 ore, but it all depends on what items you craft and what you build and when…
Some of you rewards are tied to your food storage, so higher food storage gets you more food rewards. Some of your rewards are tied to your iron storage, so higher iron storage gets you more iron. Unfortunately , more recruit storage does not increase your recruit rewards.
To answer your original question, I have two builders so I maxed my mines when I was low on iron since mines take food.