Forget the cams, a 1 meter sail difference in exactly the same wind? 5.5 gives you better speed/control in the gusts, 6.5 gives you better power. You would be better off comparing two sails of the same size, or maybe you made a typo on one of the sail sizes? I think Roger & Ken answered the cam issue very nicely.
Other than people who do very high speed sailing or racers who sail in 25+ wind speeds, not very many people will use a 5.5 cambered sail.