Bubbers you have it right
Trying to answer the question with Energy or Momentum alone, without considering the very complex subject of aerodynamics and Drag, is a very difficult thing, if possible at all.
The answer depends on about aerodynamic efficiency, and not only on the effect of having more or less mass. L/D ratio is a measure of the aerodynamic efficiency of the airplane. Similar as the efficiency of propeller blades.
An airplane flying at a less efficient AoA is wasting a larger portion of energy in the air than an airplane flying at a more efficient one.
In normal conditions, at the starting point of the experiment, the Heavy has a higher state of energy than the Light one. Due to its mass, exclusively. After we magically remove the engines and thrust disappears, deceleration starts immediately, if both airplanes keep altitude. The heavy one has more mass and , on top of that, is aerodynamically more efficient. It will then slow down more slowly. In the back side of the curve there might be cases where it would be the other way round, though.
But if we talk just about "normal" conditions, the answer is that: increased mass plus better efficiency give a slower deceleration.
Lyman
I like this thread. If you don't, why do you lose time posting here? I don't like so many other threads that I deem quite silly and I just stop reading them, like I stop reading a bad novel.