Connors could drive through the ball on har-tru, take it early, force it to the corners, overpower the opponent. But on red everything stayed up and grinding out long rallies was more to Borg's strengths.
As for comparing clay across eras, we can only really use red clay results if we want to be fair. Nadal has not played a single atp match on har-tru, so we don't know how he'd do. So ultimately it's red clay results of one versus red clay results of another. IMO.