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

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.