Is there a name for the assumption that, if two options or things are different from each other, one must be necessarily better than the other?
This sounds like a false dichotomy.
"Better" or "more good" are very fluffy, vague terms. As you said, it may be that the options are entirely unrankable.
If someone is attempting to force ranking or choice, they are committing a false dichotomy because they are neglecting option 3; that both are equal or non comparable.