Brisket... fat cap up or down?

Did mine fat side down they came out great.

The type of cooker and how the heat flows through it dictates fat-up or fat-down (also turning) of a brisket. In an Egg, the heat source is from below, so most folks cook with the fat down so it will protect the meat.

I've always done mine fat side down.

Fat cap down in the BGE
In an offset, it is usually fat cap up. Offsets pump a lot of hot smoke/air through the cooker that can result in tasty but dried out meat. Anything that can help to counteract the drying out process, e.g. fat side up, regular basting, injecting, etc., is a good thing. In addition, the heat in an offset is entering the cooking chamber from the side, not so much from below; fat cap up or down makes little difference concerning airflow in an offset. (Yes, I know about tuning plates etc.)
Meat is much less prone to drying out in the BGE, but you still need to be cognizant of airflow. Placing the brisket in the BGE, fat cap down, protects the meat from the heated airflow. This is especially true at the start of the cook when a good portion of the brisket is sticking out beyond the edges of the plate setter and drip pan. Additionally, if you baste, fat cap down allows you to baste the part you plan to eat, rather than the part you will be throwing away.