bcp,
i believe this topic had been discussed before. if i remember off-hand from what was discussed, the gliding distance for the same airframe will be the SAME regardless of it's weight. however, the best gliding speed will not be the same; the heavier one being faster. apparently that's why glider racing pilots add ballast to their glider to increase their speed!
if however, if a CONSTANT speed is to be maintained, then the heavier aircraft will have a greater gliding distance. for most airline ops, we descend at a constant speed (at least we do), that's the reason we get a very steep descent path if the aircraft is light.
hope this helps...
SR