Where it is worth more to spend the iron bars?

You can buy mining machines on the gold factory and the mine. The first one will grant you faster gold obtaining, and the second one, fast iron bar obtaining.

Since you can buy 10 iron bars with 100 gold, and taking into account the progression, what is the best strategy to maximize the iron/gold obtaining faster?

If you increase your gold production with the iron by buying mining machines, then if you need iron, dig the tunnel all of the way through until you hit the converter. Use any gold you get to purchase large quantities of iron at once and repeat the process. Works every time!