My stance on this is that IC negotiation makes for the more enjoyable experience; as you said, it leads to the more fluid outcome, with room for IC maneuvring that can in turn give an outcome that's, if not necessarily any more realistic often much more nuanced and deeper than an OOC decision of "right, let's RP so character p gets this, and character q gets that."
That being said, I can see the utility of OOC preplanning. Say, a character gets into a situation whereby it's just not fun to play them any more for the player; there, a very detached level of preplanning along the lines of "okay, this isn't enjoyable, but I think doing that would be; how can I have the character get there?" seems ideal. Nothing incredibly in-depth, but a framework.