In general it is better to reduce speed. A third option is, airspace constraints and traffic permitting, to initiate the descent earlier. A lower altitude will result in a lower true airspeed for a given calibrated airspeed. As a bonus the fuel flow will reduce as well. Winds will of course affect the outcome and can be used in your advantage. Descending someone out of that jetstream will hold him back as well.
At lower levels the fuel flow is generally lowest just above flap speed. The worst trick you can pull is to vector someone around the countryside 3000 feet and 170 knots. To overcome the added drag from the flaps needed to keep the thing in the air, a lot of fuel will be burned.