The difference in performance between the F18's I've sailed (extensively both the Nacra and the Tiger) are very very minor. Both the Nacra and the Hobie Tiger trade off weaknesses and strengths as the wind and water conditions change - even those strengths or weaknesses are exceptionally minor. There are more Hobie Tigers out there than any other F18 platform and arguably more highly skilled teams are sailing Tigers (among other things, leading to more development) so it's pretty natural that the Tigers are leading more often.