Iirc saloons are usually stiffer across the rear due to the built in strut brace you get with the body and probably why btcc used them, modern chassis are getting quite stiff now anyway so what the difference is with modern stuff I don't know, I can't see a hatch being any more stiffer than a saloon though due to the extra body that runs across the car, would be better in an accident I'd guess too due to the added strength.