Alan,
To me the Ci feels heavier (which it is). The sedan is more rigid, especailly without fold-down seats (less creaks and rattles) it maybe has a little firmer ride with SP vs. a coupe (rigidity?) The steering to me appears to be sharper and it also feels like it handles the S curves better than a coupe. I've spent sometime in a 01 330i and I can tell enough of a difference between it and my 330Ci.