Two extremes - vapor barrier vs net hammock. Vapor barrier is warmer. Take it down a notch - DWR coated vs regular ripstop. Same principle, just not as extreme.
The best way to stay warm is to minimize ALL heat transfer...since it's always transfered FROM you TO the underquilt and beyond, you're losing heat. The reason underquilts are warm is b/c the microclimes in each little down cluster are poor conductors...so they trap the heat instead of letting it out to the environment. But at the beginning, it still took that heat from your body. Whether it's conduction or convection, that's bad...so minimizing all heat transfer (in this case, especially convection) would keep you warmer. Until the condensation starts, at least...but even that's ok w/o ventilation (convection and evaporation) b/c it's a vapor barrier.
So IMO DWR coated fabrics would be warmer.
But like you said, it doesn't really make much difference...the underquilt is providing the windbreak, just as your jacket did. So as long as the wind doesn't get between your hammock and underquilt, and short of a vapor barrier hammock, it doesn't make much difference.
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IMPOSSIBLE JUST TAKES LONGER