TL;DR: If you are resource-restricted, it's most efficient to send resources and boost your least-developed village. Lower resource field levels (and culture-buildings) are the most efficient investments to increase your resource production. It is for example more efficient to upgrade 4x lvl-5 fields than just 1x lvl-8 field with the same total cost of resources.
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Early in the game when you only have few villages, you are more likely going to be "resource-restricted" rather than "building-time-restricted". This means that you can't be building in every village around the clock without queues being idle every now and then. Later on you will have enough resources to have back-to-back building queues in all the villages at the same time, and you will therefore want to be expanding all the villages simultaneously. These tables are mostly useful during those early days, and less useful later on in the game.
Below break-even points for resource fields and buildings can be used as a guidance in addition to Kirilloid's great Resource Development calculator. Tables list how many days it takes to pay back their initial costs fir increasing each field or building level. In other words, how long you need to wait until you theoretically have more resources compared to a situation where the investment would have never been done.
Practically each higher level increases the production more and more, but also costs even more to build. Therefore it takes more and more days to reach their break-even point. Earlier levels' increased population also plays larger role, for example with Iron Mine to be the least efficient resource field to level up.
If you have the Gold Resource or Crop Bonus, each of these efficiencies are about ~25% better and therefore faster to reach. Same also holds if you have for example have +25% Oases, and especially for +10-lvl resource fields when you already have the lvl-5 resource buildings in those cities. Meaning that below tables are less useful for cities and especially for capitals, where you for example want to reach as high Croplands levels as possible, almost regardless of their investment costs.
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