One common element in this generation of players is that eras greatest player is far below the eras greatest clay court player on that surface. Nadal is far and away the best clay courter of this era, with Federer a distant second best. Henin is far and away the best clay courter of this era, with Serena a distant second best. Federer overall is better than Serena on clay, however Nadal is the greatest mens clay courter of all time while Henin is not the greatest female clay courter of all time. Federer is a top 15 clay courter all time, while Serena is lucky to even be top 25 despite probably being 2nd best of a really weak womens clay court era. So ultimately which has the bigger gap in abilities on clay. Nadal over Federer or Henin over Serena.