Coupe vs Fastback

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For racing, which one is better and why?

Looks like all of the factory nissan race cars were fastbacks.
But everybody who races in ITA (dono about s13's in its, have yet to see one) runs a coupe.

Tnord and I were talking about it and just had guesses but no real answers.
Coupe is stiffer/lighter stock vs stock compared to the fastback from everything i've heard.
But given minium weights and a roll cage, it seems like they would be equally stiff and light.
So, since the fastbacks are more aerodynamic, why isn't it the choice for club racers in the US?
Only answer I could come up with is maybe the s13 can't reach its minium weight?

For the factory racing, I guess they have gobs of power and aerodynamics is more important than 30lbs or however much?

If anybody knows or cares to join in on the guessing...