"A heavy aircraft will descend faster" is incorrect - they are capable of not descending at all until the fuel runs out and the engines are running. Momentum has nothing to with that. A plane heavy with fuel will do this for a longer time.
If the engines stop, then the best glide (for distance) speed will be higher for the heavier state because the lift requirement will be higher making the drag higher and therefore the greater rate required for turning altitude into kinetic energy to be bled off via drag.
AFAIK the distance will be the same.