wagons were aerodynamically better than sedan due to the way the roof-line on a sedan slopes down in the rear. the longer the flow of air remains attached to the lines of the bodywork, the less drag will occur..so wagons typically exhibit slightly less aerodynamic drag...that's why volvo entered a wagon into the 94 btcc instead of a sedan, but of course, alfa romeo's purpose built 155 racer with wings (when no one else had them) negated the rules and required major revisions and thus the wagon was withdrawn and sedan was entered with wings in 95...

i think with the wagon...all that rattling from the rear would simply resonate back up to the front...basically a boom box for rattles...prob won't get that with a sedan.