Best efficiency will be at just below 0 manifold vacuum, without enough throttle application to cause full load enrichment. This can be at 25% throttle position, so I like to use manifold vacuum instead of throttle position, since it is a much more precise measurement of actual load on the engine. Engine load can change dramatically with grade changes, without any difference in throttle position.
In my VX, if manifold vacuum is above 6 inches, then lean burn is working, which can reduce the fuel component of the air fuel mixture from 14.6 to 1 to 22+ to 1. This may make it more effective for me to accelerate in lean burn since my consumption is reduced by close to 33% if I can use the lean burn on acceleration.
In normal non lean burn engines the 70-80% throttle position is probably your best bet for acceleration, but if your manifold vacuum is below 2 inches and your throttle position is not high enough to bring on full load enrichment then it really is not much of a difference.
If you are using higher gears for your acceleration, increasing the throttle position beyond a certain point may not give you an increase in acceleration unless you downshift. That is where the choice becomes potentially significant. Longer in the higher gear at the same load, or shorter in the lower gear at the same load. If the time in the higher gear is a lot longer than the time in the lower gear then the lower gear would be better. As the difference in time between the two gears becomes less then the benefits of shorter pulse to speed versus increased consumption during that pulse will switch from the lower gear choice to the higher gear choice.
Not sure if that explanation makes any sense
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regards
Mech