Watching a Black Templars Emperor's Champion duel a Grey Knight's brotherhood champion would be quote a sight to see. I'm bias as all hell so I'm voting for the emperor's champion. Equipment wise both combatants are equal in terms of armor, as both wear sacred raiment that is fitted and blessed to the 9's. The Black Sword is overall superior to the force sword in respects that the brotherhood champion needs to focus psychic energies to be able to have the killing power the black sword has. Both combatants are given extensive grueling training in martial prowess so I'd say their skills as swordsmen are near matched. The BC has psychic powers that he could muster to give himself an advantage but the EC has dedicated decades of his life to steel his willpower to resist effects of warp taint. If given special training to fight against psykers specifically the BC's advantage turns into a weakness. Watching the duel play out in my head I see the EC being the primary aggressor in the fight, forcing the BC on the defensive for most of the engagement. The EC would only falter if he were to become reckless, allowing the BC to exploit the most narrow of weak points in the EC's form. Other than that the BC could run himself into the EC's sword and make a last ditch martyr attempt if he was sure he would die.