Quantum ogre is more about the intent and removing actual choice from players. Good prep does away with making player choice irrelevant.
I’m assuming in your question, the first situation would have the “preplanned” fight be wherever the players decide to go since we’re discussing the QO. The difference between your two methods is in the latter, you’re making up the creature or combat on the spot by reacting to what the players have done. In the first, it didn’t matter what the players did. You were going to do it anyway, so why even give them a choice?