I've never noticed any difference in moistness between the two. Fat cap up or fat cap down is really just a function of the smoker you are using. If I'm using a traditional offset smoker, I'm going fat cap up because that protects the meat better. If I'm using a Traeger, I'm going fat cap down because that protects the meat better.
For me, the end product is mostly a function of how low and slow you are willing to go. I will start my brisket at 180 degrees for the first 4 hours or so. I'll then go to 225 for the remainder of the time. I will NEVER wrap a brisket and I will spritz with 50/50 water and apple cider vinegar every 30 minutes through the entire smoke, which will typically take 24-26 hours. Yes, I get up every 30 minutes through the night to spritz. It's worth it. I also always rest the brisket for a minimum of 8 hours. 12 hours is preferable though.
