If you can only get one, which would you prefer? An apology, or someone understanding why they should apologize?

The understanding. I can't stand it when people say sorry without meaning. It's just a word then.

It’s kind of hard to say. Someone apologizing just to keep the peace, or offering the "non-apology" apology ("sorry if that upset you") is pretty useless in terms of rebuilding any kind of relationship.

But the person who knows they were in the wrong and can’t apologize, clearly doesn’t really want to rebuild it.

Understanding

is it even a real apology if they don't understand they were wrong?

it doesn't matter which in that case, equally worthless to me

how would you ever know the latter without the former?