Does not matter so much to me. If I know I am re-heating I wrap it in a foiled pan which I have put in a small amount of liquid. Beef broth, lea & Perins and some original rub or maybe some thinned sauce if the brisket is built that way. Not alot just enough for moisture.
I usually will reheat in the oven, my oven will cook as low as 175 degrees. But if I need the oven, the smoker/pit is always available. You don't need to get it hot, just warm it because brisket hot is a pain in the derriere to slice.
People will not realize its not hot and they will like it better when able to taste the meat without removing a layer of hide off their tongues. Other countries know this better than we in the US because they have not trained their mouth on hot fast foods. Seriously think on it, do you normally like that cold hamburger or fried chicken or burrito? Its because the heat hides some of that taste.
Warmed not hot, and let 'em know why, because as a pitmaster to get food to taste this good you have to wait for the meat to tell you when ITS ready. And you are smart enough to know that.