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It's not frontal area, but cross section, at the point where the area is greatest. Obviously, for anything except maybe a cabover semi, the area is greatest about where the driver sits, and much less at the front.
As for which matters, both are equally important. Drag depends on the product CdA. When designing or choosing a car, it's probably easiest to change A: you just buy e.g. a Miata instead of a Suburban.
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