This means when the worse cards are in play, you should buy the ones that will be the same.
That doesn't make sense. If they're worse, then they aren't the same.
Age I brown cards, Age II brown cards, and Age I silver cards are incomparable; no one is clearly better than the other. Yes, Age II brown cards give more resources than Age I brown cards, but an brown card obtained in Age I can be used to buy things for two more ages, while a brown card obtained during Age I can be used to buy things for one more age (and if you buy military in Age I, that military gives you benefits for all three ages). Silver cards can be used to buy high-value cards, but those cards don't show up until later ages; a silver card obtained in Age I can't be used to buy anything in that age.
Your argument, which you aren't articulating completely explicitly and so I have to read between the lines a bit, is that if you first have a choice between one resource of Type A or one resource of Type B, and then later a choice between two resources of Type A or one resource of Type B, then you should first choose Type B and then Type A. While there are some valid intuitions motivating that reasoning, it is not a conclusive. One motivation would be the assumption that the game makers designed the game to be balanced, so if they give you a choice between one of Type B and two of Type A, then one of Type B must be worth two of Type A. But the very fact that you think there is a clear strategy means that you are implicitly accepting that it isn't completely balanced. Furthermore, you are assuming that the relative worth of the two types of resources are the same in both ages, so if one silver resource is worth two brown resources in Age II, then it must be worth two in Age I. But as I argued above, that argument does not hold, because there are things available in later ages that require silver resources, and thus silver resources are worth more in later ages.
So it costs a coin, big deal.
You can't spend coins on resources unless your neighbors have those resources.