While I don't think anyone has specifically asked this, I believe that one issue informing the debate is the question: "Who has a greater impact on the overall film?" -- Note, that even if we came up with a clear answer to this there'd still be a distinct question as to how much that should matter in parceling out blame, praise, and bucks.
Initially, to me, and certainly to my pre-screenwriting, merely a film-fan self, it was the director. I'd track movies by director since the director would often be a good predictor of whether or not I'd like the film. Actors would be a good indicator of whether or not I'd like a particular performance in the film -- but it doesn't matter how cool Samuel L. Jackson is, I'm still not going out of my way to see a drama about adopting crack babies.Moreover, there's a logistical reason the director has a greater impact -- the director comes in later in the project and controls it largely to the end. For a sort of analogy, imagine that you split the driving with your friend across country. If your friend does the second half, it'll be the friend that controls whether you end up in
That said, there is a sort of special kind of control you get by going first. If it's a spec, you control the whole thing really: what genre, types of characters, type of ending, and so on. Any changes made will be made off the basis of what you wrote. However, this is a strange sort of control, since your spec will only be picked up if producers want it, and then only by producers who are looking for that kind of film -- the producers act like a filter and that filter has a huge impact on the kinds of movies that eventually get made.
Even in assignments, unless the first early screenplays are completely scrapped, the approaches used there color the rest of the production.Anyway, this sort of pure theory can be fun, but if you really want to find something out you need to do empirical experiments. So, what I decided to do is look through my collection of favorite movies and for each one compare the list of others films the director has directed, and the list of other films the writer has written.
In essence, run a test to see how good the writer is as a predictor of whether or not I'll like a film.
Note that this isn't exactly fair as a means of determining contribution -- in part because of the "doing the second half" rule above meaning that a sloppy director can mess up a good script, moreover, a director gets to do his own filtering and better directors will only choose scripts that are already pretty good, or have potential.
I run the test in a separate post that you can link to here.