Always 1000% fat up for me!
I started off doing briskets fat up and then with all the talk about fat down protecting etc., I decided to give it a try fat down. First one I did wasn't close to my fat up, so I figured it was something I did so I gave it a shot a couple more times with similar results. So now I stick to what works and fat up, fat up, fat up is my preferred and only method!
FWIW, I've done them both ways on my Gator Pit and Primos and the fat side up always comes out superior to the fat down!
Have I mentioned I like to cook my briskets fat side up?? Sometimes I have a problem making my self clear!