At first blush, the right to acquire a scarce resource for only two units of your "abundant" resource is an advantage over having to trade four units of the same resource. But because of "side" conditions, that is not always true in Catan.
There are five resources. Call them A, B, C, D, and E. Let's say that E is the scarce resource. You can either build a road followed by a port settlement to access either a "2 to 1" harbor or else you can trade at a "4 to 1" ratio (these are references to the required payments for the scarce resource ) with no strings attached. The "catch" is, at the "4 to 1" rate, you can trade any of the four other resources at that ratio, whereas at a "2 to 1" harbor, you can trade only one specific resource, say, D, for E, at the favorable 2 to 1 ratio. On the other hand, you do not have to build a port settlement to trade at the 4 to 1 ratio, whereas you need to do so to trade at the 3 to 1 and 2 to 1 ratios.
Maybe the question resolves into the following: You want to trade for E, and D is "only" your third most abundant resource (out of four), with A being very abundant, and B being more abundant than D. How do players determine whether they should aim for the 4 to 1 "straight up" ratio where they can trade A or B (or even D) at that ratio for E, versus needing to build a port settlement where they can only trade D at the ratio of 2 to 1?
Note: Someone pointed out that the original question about the comparison of a 2 to 1 versus 3 to 1 trade ratios was a duplicate of another question. I had planned a follow up question about the comparison of the 2 to 1 trade terms versus the 4 to 1 trade terms, and reworded the original question into this "followup" form, which I believe is not a duplicate. One important difference with the other question is that you do not need to build a port settlement to trade at the 4 to 1 ratio, whereas you need to do so to trade at the 3 to 1 ratio.