Having just rewatched TWOK Director's Edition last night, it seems clear to me that it's a mixture of both motivations. I still think he had a plan (he was
trying to convince Khan to go away because Enterprise already had orders to bluff Khan into thinking they needed longer to repair themselves, and playing to Khan's ego was a way of making Khan think he'd won), but the heat of the moment and the following scene where he discusses with Dr. Marcus about how Khan "wants me dead" indicates that the pressures of the situation are
getting to him. It isn't until he's in the Genesis cave that he reaches a state of perfect calm again.