used by a skilled rider, the isonic is faster than the futura in all conditions. however i did manage to push my 93 futura to 32.86 knots. the isonics really arn't hard boards to ride, and i dont see why people create this phallacy that they are. for an advanced rider (ie planing gybes on both tacks with good consistency) the isonics are great boards and are more fun for gybing than the futuras because they carry more speed throughout the turn and tend to 'slice' through the water than carve like a futura. if you have a quiver of race sails or 'recreational' race sails (eg a neilpryde rs slalom, 3-cam) the isonics match much better than the futura for those sails. once up to top end speed the isonics literally get to a point where they just fly over chop instead of going through it, because of the lower noce rocker, but you need to be able to get there though. the isonics can point higher upwind and keep more speed at that angle than a futura.
the futura does have some advantages though. for people learning to carve gybe for the first time the futuras tend to keep going a bit better at slow speeds and are more forgiving of dodgy technique. the futuras have a softer ride over chop going at normal crusing speed. with the futura you dont have to be as foccused on the chop going at slower speeds because of the slight extra nose rocker wheras with the isonic you do have to make sure the rails up front dont catch.