The most efficient flight from a fuel consumption point of view is actually a very slow flight - you should fly the whole flight - climb, cruise, descent - at best L/D speed, which will start slow and will actually get slower as you burn off fuel. In the original question choices that would recommend a steep descent with low power, but would also recommend a tediously slow cruise.
But in all the aircraft that I am familiar with it is not the fuel flow that is the dominant cost of flying - the dominant cost is the hour meter (which determines the maintenance costs and the airframe depreciation).
In other words, if by "efficient" you mean least total cost, then you should always go as fast as you can. That still means staying high as long as you can (to get the benefit of favourable TAS/IAS ratios) but then descending at red line, with as much power as you need to stay at that speed. So - neither of your two original choices!