So which one is the truth, do you need to plan more track miles to descend a heavier aircraft or a lighter aircraft?
If you descend at the same speed in both, then you will need more track miles in the heavier aircraft. Simple explanation: best L/D speed increases with wing loading, so your glide angle will be better in the heavier aircraft == more miles.
Additionally in terms of slowing down, which do you need to plan more track miles for? A lighter aircraft will take time to slow down due to its inertia. But a heavier aircraft will also take time to slow down because of its momentum, sometimes you need to drop the gear for example.
Again, a heavier aircraft needs more track miles. Simple explanation: as you are operating in a better L/D regime as you slow down, the deceleration will be less == more miles.