The mental process I go through to determine my preferences is highly scope-sensitive.

For example, the process underlying asking "which of the choices I' have the practical ability to implement right now do I prefer?" is very different from "which of the choices I have the intellectual ability to conceive of right now do I prefer?" is very different from "do I prefer to choose from among my current choices, or defer choosing?"

Also, the answer I give to each of those questions depends a lot on what parts of my psyche I'm most identifying with at the moment I answer.

Many of my "should" statements refer to the results of the most far-mode, ego-less version of "prefer" that I've cached the results of evaluating. In those cases, yes, "should" is equivalent to (one and only one version of) "prefer." Even in those cases, though, "prefer" is not (generally) equivalent to "should," though in those cases I am generally happiest when my various other "prefers" converge on my "should".

There are also "should" statements I make which are really social constructs I've picked up uncritically. I make some effort to evaluate these as I identify them and either discard them or endorse them on other grounds, but I don't devote nearly the effort to that that would be required to complete the task. In many of those cases, my "should" isn't equivalent to any form of "prefer," and I am generally happiest in those cases when I discard that "should".