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The answer will depend on how long you will be travelling at the higher speed.
For a given target speed the Kinetic Energy (KE) embodied in the car at that speed is the same whether you accelerate slowly or rapidly. If you accelerate quickly, there is some small (extra) loss due to the higher average velocity. This diminishes in relative magnitude the longer the time spent at the target speed.
Against that the engine is more efficient when loaded up to the point you get into closed loop fuel enrichment. That points towards the faster acceleration being more efficient.
Inlet manifold vacuum is the best way to determine engine load, rather than throttle opening. At low engine power output (i.e. rpm) the engine can be the ultimate restriction on airflow, not the throttle i.e. full engine load is reached at some fraction of WOT.
Further opening the throttle after that point is reached will not increase the engine power output and you will be in open loop mixture control at significantly less than WOT.
Using all of the gears allows the engine to be operated near its peak efficiency for more of the time than does skipping gears.
If you wish to accelerate - on average - more slowly, accelerate fast and shift slow (coast in between gears).
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