The Phantom was singularly obsessed with causing as much death and bloodshed as they possibly could no matter the method, whereas it seems like Sar'resh simply lived for the joy of fighting and being able to use his martial skills. So I'd imagine there was some tension/condescension of approach; The swordsman would look down on the assassin for skipping all the fun bits, the assassin would disdain the swordsman for wasting time before getting to the point.