Fincher, by far. While, i like Cameron a lot, he's more of a blockbuster guy, when it comes to the simple act of staging a scene, he crafts a lovely piece of drama, epic sci-fi scenes and set pieces, and camera motion.
Fincher is also a master of cinematography, using a color palette like no one else, the dark, grey and blue tones, whereas Cameron has a good but not memorable visual style with orange or blue, despite having good cinematographers.
For the films themselves, I wasn't a fan of Panic Room or Benjamin Button, but i love Seven, Gone Girl, Zodiac, The Social Network... and found The Game interesting, from a directorial standpoint.
I have to be honest and say that of the Cameron' films I've seen all of them except his documentaries. I loved Terminator 1 and 2, Aliens was great, True Lies was fun and i was mildly entertained by Titanic and The Abyss. Avatar was meh.
This video is probably surface level, but shows his incredible skill and how his films are stylized.
To sum up, Fincher is an interesting filmmaker, and even though I can't say he is the best, I actively think Cameron's directing while being completely focused and excellent in action scenes, doesn’t have the same artistry or technical craft. Fincher creates iconic images, while Cameron has yet to compose a shot I can even draw up from memory.
I am shocked at poll results.