This is a trick question. The answer is:
None. The 28 spoke wheels will be heavier. Here's why -
Keeping all else equal, eliminating spokes reduces not only weight, but also stiffness and durability. To gain back that durubility, you have to use a stiffer rim, which will add weight back. The weight needed to added back to the rim will be greater than the weight eliminated from the spokes.
So your saying that when Mavic makes a rim that can be drilled for 20 or 24 holes, or 28, that they are changing the manufacturing design, and adding more material into the 20/24 holed rims than the 28 - rather than just changing the build pattern? I don't think so.
What your saying is true technically, but probably makes no difference in the real world.