Neither. I take some of the juices that the brisket throws off, defat them, add a bit of good beef stock and a little butter, and emulsify with a touch of Dijon. This I paint on the slices (both sides) as I slice.
If you're looking to boost extant flavors try an injection. A suggestion: start with a good beef stock that you can reduce or, barring that, a 75-25 mix of quality canned low-salt chicken stock plus quality canned beef stock (straight canned beef is too metallic; don't worry, the chicken will add meaty notes, not chicken-y ones). Boost meatiness/umami by adding dried mushrooms and add complementary notes and a bit of sweetness by adding aromatics (onion, garlic, shallot, a teeny piece of ginger perhaps), and add a little rub (without sugar or salt). Go on the lighter side with the aromatic and rub additions as you'll want to blend all then reduce in a pot till a good level of saltiness is achieved. Cool, strain out the solids, then tighten with xanthan or emulsify with a little clarified butter, whole butter, or oil, and a touch of Dijon.
Just a thought.