I get tied up in this one, too, because I'm an emergency doc, which implies a bit of seat-of-the-pants decision making in a very unplanned situation (INTP), yet what I actually love is rapidly processing the flow of information into a diagnosis. I'm like House on amphetamines - 'If I don't figure this out in the next ten minutes this person will die' sort of stuff.
However, if equipment fails or isn't ready because someone hasn't done their job and checked in advance, I will be inwardly seething, and will never trust that person ever again.
Which I guess is more INTJ.
The exam preparation is a good discriminator, too. I can't imagine feeling comfortable with 15 minutes of revision! Perhaps the question that separates P/J is 'when you're in a situation where planning would be a very good idea, are you still comfortable if you don't plan?'