I think this talk of efficiency is rather vague and largely unimportant to the average commuter, especially when it's on the order of a few percent.

As far as I can see, both have practical advantages:

IGH:
-chains can last longer, since full-bushing chain can be used with a straight chainline, and can be fully enclosed in a chain case.
-use of said chain case prevents grease marks on clothing, making the bicycle more suitable for commuting in everyday clothes.
-can shift while stationary.
-simple operation due to only one shifter.
-generally less demanding of maintenance.
-rear wheel requires minimal or no dish.

Derailleurs:
-potential for a wider range, and smaller steps within that range
-can shift under load with more modern systems.
-can customise gearing to a large extent, unlike IGH, where the steps are fixed and only the 'final drive' ratio can be changed.
-simpler to repair at home.